5punk

As requested by Grimmie, I'm moving my big tactics post here, and will update it for all possibilities.

Terran

Important points
Always research, it's the key to success. If you grab MMMs, you need to get combat shields, stimpacks and concussive grenades. Siege Tanks obviously need siege mode, Hellions are useless without the bonus damage against light armor. The Thor's bombardment can take out a large health pool unit very quickly. Battlecruisers are only marines with a high health pool if you don't have Yamato Cannons. Banshees are useless without cloak, if you have a Raven, a well-placed Seeker Missile can take out a large group of units, and Ghosts need invisibility to be useful.

Hotkeys. Hotkeys hotkeys hotkeys, and control groups. Hotkey usage adds up and you can get over 5 minutes ahead of clickers by the end of the game.

Terrans are very dependent on being able to move out with an effective horse, fast. You'll have to expand in 5-8 minutes. If you can't do this, you're doing something very wrong. You'll have to be able to push out units fast, if you're doing an MMM, be sure to have at least 2 techlab and 2 reactor barracks.

Finally, Terrans have the most harassment potential, which you can use against unprepared opponents to gain a heavy advantage. For example, you can consider building up a small horse of reapers with nitro packs and micro them (again, hotkeys and control groups) around cliffs to kill workers - if you're lucky enough, you might also get a few buildings. Vikings can fly in and transform as well, they're tougher than reapers and their stance changes enable them to escape from a wider range of units than reapers. Ravens can deploy auto-turrets behind workers and seeker missiles do heavy damage as well. Banshees can fly in cloaked and kill workers while the detectorless opponent capslock swears in All chat.

Early game
Terrans start with 1 extra supply capacity, which doesn't factor into the standard build order, at all. Terrans, however, require Vespene Gas much, much earlier than Zerg or Protoss. The most important thing, though, is a constant flow of workers. Your Command Center should not have any idle time until you have 18-20 workers. Do not queue, though, as it's a waste of resources.
A fundamental defense mechanism of Terrans is blocking the choke, as you can lift Barracks and lower Supply Depots on demand, it's a useful thing to do. All chokepoints can be blocked by 2 Supply Depots and a Barracks, except Scrap Station, where you require 3 Supply Depots and a Barracks.
Build your first supply depot once you have 9 workers. Immediately after it's done, build your first Barracks, and once you have 100 spare minerals, build your second Supply Depot, blocking the chokepoint. Spare up another 75 minerals and begin building the first Refinery. Set 3 SCVs to harvest it, meanwhile your Barracks should train Marines. At 25 gas, attach a Tech Lab to the Barracks, and while it's building, upgrade your Command Center to an Orbital Command and build the other Refinery. At this point, you should decide what tactics you're going to use and whether or not you will reaper harass and proceed accordingly.

The chokepoint block.

Scouting
Terrans employ three types of scouting.
Worker scouting: The worker scouting is fairly simple: send a worker into the enemy base and set it to patrol. The significance of this practice not immediately noticeable, but you can pick up a fair bit of information if you look around. If a terran's missing his Command Center, chances are he moved to a rich, if a zerg's not building any drones, he's 6 pooling, and if a protoss puts down two gateways instead of one, chances are you're going to have your face rearranged by zealots shortly.
Scanner sweep: Terran scouting is the safest and easiest task once you have an Orbital Command. Just pop a few scanner sweeps into the enemy base once in a while and you'll have all the information you need.
Xel'naga Tower: Another universal scouting method manning the Xel'naga Towers. It's usually an early warning of an incoming attack, which gives you time to prepare.

Main Terran tactics
MMM: One of the most popular Terran tactics. Takes advantage of the fact that Marines and Marauders can deal with a wide variety of units, and it's especially lethal to armored ground and single-target air units. Its main weakness is AoE attacks, since both Marines and Marauders have a relatively low health pool. Its usefulness peaks in mid-game. Units involved in this tactics are Marines, Marauders and Medivacs. The most effective way to get a formidable ball of MMM is 2 Tech Lab Barracks, 2 Reactor Barracks, 2 Engineering Bays and 1 Reactor Starport.
MMM Siege: An extension of MMM, which uses the Factory you need to build to get a Starport to train Siege Tanks. It's very useful against certain types of units, such as a massive group of Hydralisks, a large ball of MMM, Zealots without Charge. It's also great if your opponent has base defenses, as you can take them out without getting damaged.
MMM Hellion: Another extension of MMM, which uses a Reactor Factory to push out Hellions. This is a situation tactic, it's very useful against Zerglings and pure Marine rushes, but you can also build on the speed of the Hellions - it can kill a group of Roaches unharmed if you don't fight on creep.
MM: A rush tactic resembling MMM, however it scraps Medivacs entirely. It's effective against Zerg players who skip Banelings altogether and Protoss players who don't go for fast Colossi. The tactics involves pushing out a ball of Marines and Marauders and attempting to kill the enemy as soon as you get Level 2 infantry weapons and armor.
Mech: The mech tactics takes pressure off the Barracks and places it on the Factories instead. It involves getting a smaller group of MMs, then mass producing Vikings, Siege Tanks and Thors. This is a very effective build if your MMM is being countered. Effectively, this requires 1 Reactor Barracks, 1 Tech Lab Barracks, 3 Tech Lab Factories and 1-2 Reactor Spaceport(s).
Early Rich: This is a high risk-high reward tactics. You can exploit the fact that the Command Center is mobile and immediately move it to a rich mineral field. It'll get you started faster than the opponent, however, he'll more likely try to rush you than not. A very effective way to use this tactics is to build your base back at your starting location and block the chokepoint, man a Xel'naga Tower and at the first sign of trouble, head back.
Proximity Bunker: aka. The Why You Should Scout Your Own Base aka. The Tactics Gunslinger Would Use If He Played Starcraft. Hide an SCV in the enemy base and make sure he doesn't find it. Once your Supply Depot has been build, build your first Barracks in the enemy base, and when it's complete, a Bunker. Man it with Marines and build Bunkers closer and closer to the enemy base so that the previous Bunker kills anything that tries to destroy the one you're building. This is very cheap and doesn't work in higher tiers, though, so use it sparingly.

Zerg units as viewed by a Terran:
Zergling: Probably the most useless unit in the game, even when fully upgraded. A horde of zerglings can be disposed of in a wide variety of ways: Stimpack Marines, Hellions, Siege Tanks, Assault mode Vikings, Ghosts, Reapers, you name it.

Queen: Yes. Queens. You will very rarely see Queen rushes, it's possible, though. Queen rushes can be easily countered by all units, but it's important to focus the same target to quickly waste their transfusions.

Overseer: Overseers are not attack units, but they're worth mentioning, as they have two very useful abilities and one very useful not-quite-an-ability. They can spawn changelings which take the shape of your marines to scout your base, and they can temporarily disable a structure, suppressing its ability to train and research. They might also be strategically positioned outside your base to provide vision, as spawning nydus worms require visibility of the target location.

Baneling: These do AoE damage not only when they explode, but when they're killed as well. Very hard counter to MMMs - most semi-competent zerg players will either send these at you in hordes with speed upgrade, or they'll keep them burrowed outside the choke. Since all ground zerg units can burrow, it's generally a good idea to have a raven. Use the raven's auto-turrets or a few deployed siege tank to pick them off from a distance.

Roach: Roaches are another counter to MMMs, particularly the marines. Marauders will tear through them, although with heavy losses. It's generally a better idea to micro hellions and use them to kill the roach group without any losses. In a large fight, you probably won't take on the roaches head-on - they can move while burrowed and will probably pop up near your siege tanks or marines.

Hydralisk: Hydralisks are the marines of the zerg, they're general purpose ranged units. Don't bother trying to use air units with a load of these around. Thors will also fall within moments after the focused hydralisk fire begins barraging them. The range upgrade gives them an edge, but they're very vulnerable to siege tank fire. Zergs using a combination of roaches and hydras will first move the roaches in, soon followed by heavy fire from hydras.

Infestor: Infestors are the energy users of the zerg, sporting a wide variety of evil abilities. Their fungal growth will immobilize and slowly kill your MMs, and their neural parasites can take control of your most dangerous units. If you find your own siege tank firing at your MMM ball, there's someone on the other end of the ginormous tongue hanging out of it. Their infested terrans are great harassment units as well, and they can deploy the eggs while burrowed. Like roaches, they will move into the fight while burrowed. In short, the infestors are a soft counter to all tactics, but can be rendered useless relatively quickly by detectors and ghosts.

Mutalisk: Multi-target air units. In large groups, they're great against MMMs and vikings, but the Thor's air splash will take any amount of them out in mere moments. If the zerg is not using roaches to take out your siege tanks, they'll probably sneak their mutalisks in through the back. Be sure to cover your SCVs, as mutalisks can fly in and destroy them before you have a chance to react.

Corruptor: Anti-air flyers. They can also apply a single-target DoT, which can cripple your thors very quickly. Their attacks take down battlecruisers and banshees effectively, vikings moderately effectively. Your best bet to kill these is marines, lots of them.

Brood Lord: Anti-ground flyers, the ultimate counter to MMMs. A few of these is a joke, but once the zerg has 4-5, you will have trouble killing them. They consume less supplies than most top-tier units (4, as opposed to the battlecruiser's 6), but take more time to produce. They spawn broodlings when they attack, so it's crucial to focus fire them. However, your best bet at taking them out are vikings and the battlecruiser's yamato cannons.

Ultralisk: Ultralisks are heavily armored melee units which do splash damage. They'll take out a ball of marines or even an MMM rapidly. Aside from the obvious air solution of removing them from the combat, you can also use stimpack marauders to micro them to death.

Protoss units as viewed by a Terran:
Zealot: These shouldn't be a problem. Some Protoss will try to rush you early with them, but if you block your choke properly (Depot - Barracks - Depot), you can take them out with marines easily. In open fights, they tear through your marines, but if you have stimpacks and medivac backup or marauders with concussive grenades, they should go down pretty fast. Siege tanks also thin their ranks, but they're only useful until the protoss researches charge.

Sentry: Psionic fighter, which does quite a bit of damage, but it is a glass cannon. A few well-placed horse fields can block a portion of your horse off, or in a large fight, it might horse your units to enter one-by-one, giving the protoss an advantage. If you see it putting up horse fields, your best bet is to retreat and fight elsewhere or wait for the fields to expire. Another ability they have is the shield, which reduces all incoming ranged damage, effectively it cancels out two levels of weapon upgrades for marines. In this case, they should be taken out first. You really don't need anything specific to counter them.

Stalker: Very lethal in a large group, also great for sneak attacks as they can blink up to your base if something provides them vision (eg. Observer). They can and will make a smoking pile of rubble out of your siege tanks and they'll do heavy damage against your air units as well. Marauders will pretty much destroy them, though.

Immortal: The arch-nemesis of ground mechs - they cut through thors and siege tanks like hot knife through butter. If your opponent is massing these, an MMM build destroy them pretty quickly. They're also very vulnerable to banshees and battlecruisers, but using them against immortals is a waste, since as long as the immortals have shields up, they will only take a maximum of 8 damage.

Colossus: Also dubbed the glass death star. With zealots and stalkers in the front line and these in the back, your MMMs will turn into mere Ms in a few moments. You have quite a few options for taking them down, though. Vikings are the most effective, but thors with their raw firepower and siege tanks with their enormous range also make them collapse in mere moments.

High Templar: A hard counter to MMM. If the enemy has a few of these, you'll need the micro skills of a korean to win the fight with MMM. They're moderately less useful up against mechs, but their feedback ability will burn their energy reserves and do some heavy damage. Siege tanks can take them out from a distance, though, or alternatively, a ghost's EMP round will simultaneously remove their shields and energy, rendering them useless.

Dark Templar: The bane of the unprepared, otherwise generally useless. They're permanently cloaked, so either bring ravens or use your orbital command's scanner sweep to reveal them temporarily.

Archon: Very powerful melee unit, they'll kill most units they get close to, ground or air. Siege tanks should thin their ranks before they get to you, and since they're literally all shields, ghosts using EMP rounds will kill them in moments.

Phoenix: These can and will destroy your air units in mere moments, unless you can sport a larger horse of vikings. Their graviton beam ability can take out siege tanks as well with relative ease, or in a larger horse an equal portion of your infantry, depending on your opponent's clicking speed. Thors are immune to their disable though, and their air splash should destroy them in mere moments. Marines are a bit more vulnerable when up against them, but if your marine horse is at least twice as large as his phoenix horse, it should be enough.

Void Ray: Near-useless alone, deadly in large groups. Void rays depend on being able to fire on the same target long enough to get supercharged, therefore battlecruisers and thors are very vulnerable against them. A large horse of thors should be able to air splash them to death very quickly, though, but if that's not available to you, just use a large ball of marines with medivacs.

Carrier: General purpose rapists. Lined up behind cannon fodders, these will wipe you out. Generally, you'll need to focus fire these first with thors, or second after colossus with MMMs. Your best bet at taking these out, though, is battlecruisers with yamato cannons or a load of vikings.

Mothership: The mothership is dangerous because of its abilities. It cloaks all nearby units, it can teleport new units into the fight and it can create a black hole, temporarily sucking in nearby units. This is pretty much the first unit you should take out, mainly because you cannot see any other units. A very important thing is - if it creates a black hole on a group of your units, move all your units inside the black hole. While they're inside, they're invulnerable but cannot attack, so this way, you preserve your whole army.

Zerg

Important points
Always research. I mentioned that it's important for terrans - it's critical for zerg. Most zerg units are useless without their respective upgrades.

Hotkeys, as mentioned in the terran guide.

Zerg tactics are usually one of the two extremes - either rush or turtle until you hit 200 supplies. Zergs have the hardest time pushing units out, as you have up to 3 larvae available, and it's very important to build 20-24 workers as soon as possible, therefore, zerg is very vulnerable early on - it takes 2 zerglings to kill an unmicro'd marine, 3 to kill a micro'd one, 4 to kill a zealot. Therefore it's usually a smart move to train a few of your only larvae-independent attack unit - the queen. Doing this early has quite a few advantages - you will need multiple queens to keep a high larva supply in your 6-8 hatcheries, and by the time you need these queens, they'll be on full energy. In early rushes, these queens can also transfuse eachother, or alternatively use their excess energy to expand your creep.

Creep is a restriction and a buff to Zerg. Fighting on creep gives you a movement speed advantage, which is critical for getting often-focused hardhitters into the fight, such as an ultralisk. Use your queens' excess energy to plant and spread creep tumors.

The early harassment potential of zerg is very low. Your effective harass units are late tier 2 - mutalisks and infestors. The earlier is a great hit-and-run attacker, the latter can indirectly harass with infested terran eggs without ever leaving invisibility. A less effective harass unit is the roach, which can tunnel into the enemy base, but it's not hard to pull it off. Your best bet at roach harassment against terran is to wait until he opens up his choke and crawl to his orbital command. If you manage to destroy it before he kills your army of roaches, burrow to regenerate them. For protoss, this tactic may not work very well, as they would have observers out before you could get your roaches in.

Hatcheries are the most important buildings for the Zerg. If you have 300 spare minerals you cannot spend elsewhere, whip up another hatchery. If your army is wiped out late game on the offensive, chances are your opponent will try to do a counter-offensive attack. Six well-fertilized hatcheries have 7 larvae each, which means you can push out a formidable army, 42 hydralisks, before he gets to your natural expansion. If you select all your hatcheries and bind them to a control group, you can macro while not attending your base very easily. You can get heavy reinforcements in the middle of a fight simply by pressing 1, S, then holding H.

Queens are your most important units in terms of economy. A queen spawns additional larvae for hatcheries and regenerates energy at a rate so that they can keep one hatchery permanently larvaed. In practice, you probably won't be able to macro that well, so the general rule is that one queen maintains two hatcheries.

Early game
Zergs require Vespene Gas later than Terrans but earlier than Protoss. It is crucial to get a good amount of workers going, or you'll struggle in the resource production department. Be sure to replace the workers you use for construction, as they are lost in the process.
Zerg have the least base defense potential, as covering the chokepoint is effectively blocking you in and you cannot do it very early as you need to maintain creep under your buildings or they will slowly decay. The zerg defense buildings have one very useful ability though - they can be uprooted and moved. This is not a strictly defensive skill, though, as they can be exploited in rush tactics as well.
There is no universal early game build order for zerg, therefore it's important to worker scout your opponent.
The most economically effective build order is spawning an overlord at 9 workers and building your spawning pool at 12-13 workers. This is commonly called 13-pooling. On the long run, this gives you an economical advantage, however, it cripples you against rushes.
The first level of safety-for-economy trade-off is 10-pooling. Do this if your opponent is up to something fishy, such as immediate double gateway. Spawn drones exclusively until you have 10 of them, then gather until you have 200 minerals and build a spawning pool. Afterwards, build an overlord and continue your worker production.
The second level of trade-offs is 8-pooling. Use this sparingly, as it hits your economy hard. Spawn drones until you have 8 of them, harvest until you have 200 minerals, build a spawning pool. If, for some reason, you need to get zerglings out quick, spawn an overlord and 2 drones while the pool is under construction and you can push 6 zerglings out immediately after it's done.

Scouting
Zergs employ a wide variety of scouting techniques.
Worker scouting: The worker scouting is fairly simple: send a worker into the enemy base and set it to patrol. The significance of this practice not immediately noticeable, but you can pick up a fair bit of information if you look around. If a terran's missing his Command Center, chances are he moved to a rich, if a zerg's not building any drones, he's 6 pooling, and if a protoss puts down two gateways instead of one, chances are you're going to have your face rearranged by zealots shortly.
Changeling: The changeling scouting is a medium risk scouting method. Drop a changeling with an overseer into the back of the enemy base, or if he's proven to be slightly visually challenged, drop it anywhere and move it up the front ramp. The advantage of this method is that you'll be able to hide in plain sight and even if the changeling is lost, you lose nothing. The disadvantage is that it's easily perceptible and it might prompt your opponent to change tactics immediately if discovered. He might also get a little more alarmed, which is enough for you not to be able to move changelings to his main army.
Speedling scouting: If your opponent proves impervious to changeling scouting, you can consider using your otherwise cannon fodder zerglings with speed upgrade. Ram one of them into his main army, which gives you 3-5 seconds to see what he's got.
Burrow scouting: Another medium risk scouting method. If your opponent doesn't have detectors scattered around his base, you can move a roach under his main army to check what he's got.
Xel'naga Tower: Another universal scouting method manning the Xel'naga Towers. It's usually an early warning of an incoming attack, which gives you time to prepare.

Main Zerg tactics
Ground Mix: A great solo tactic is building up a four unit mix of banelings, roaches, infestors and hydralisks. If your opponent has detectors at his base, you will have to take him out on open ground. Your roaches and infestors should go in first, using roaches as cannon fodders, pop up the infestors and spam your abilities. Send in the banelings and use the hydralisks to do damage from a distance. Alternatively, you can move all these units into the enemy base through a nydus worm.
Mass Mutalisk: If your ground mix is unfeasible, another option is to spawn a horde of mutalisks along with some sort of ground cannon fodders and infestors.
Air Mix: Heavy turtling tactics, which involves rushing to get corruptors and brood lords. Add infestors to the mix and the build is mostly unstoppable - the only units which nuke these are Thors, Vikings and Phoenixes. The earlier should be parasited, the latter two should be fungal growth'd, which makes it a lot easier to deal with them. On it's own, however, chances are you will get steamrolled before the brood lords can roll out. If you plan on using this, it's better to start with a Ground Mix tactics, then proceed to slaughter your army and replace them with corruptors and brood lords.
Baneling Drop: Semi-cheap tactics involving massing banelings, which are then dropped into the enemy base via overlords and nydus worms. Banelings can cause truly terrible, terrible damage to the enemy base, a well-placed baneling drop can take out a headquarters and all the workers or a ball of MMM or a protoss ground mix.
6-pool: aka. The TezzRexx Way To Get Into Gold League. Cheap noob-slaughter tactics, which is pretty much a gamble. Don't train anything at start, just mine with your 6 drones until you have 200 minerals. Build a spawning pool and train 2 drones while it's building. Train 3 batches of zerglings when it's done and attack the enemy base. There are three possible outcomes: 1) You will hit a terran choke block. If there are no marines on the other side, try to destroy the blockade before the first marine is completed. If there is a marine, retreat and try to win with your heavily crippled economy. 2) You gain access to the base, destroy the nearly completed unit production facility, the headquarters and all the workers. You won, I hope you're proud of yourself. 3) You gain access to the base and the enemy workers attack you. You have two options here, either retreat or micromanage. The latter is the better option here, as if you manage to evade the workers, you can destroy the enemy's unit production facility, as the opponent will move his workers back to salvage his economy loss he acquired while no workers were gathering. You should continue to push zerglings out, as from here on, you need to keep the pressure on him. If the pressure isn't pressing enough, your opponent will push units out fast enough to counter you.

Protoss units as viewed by zerg
Zealot: If zerg is butter, zealots are hot knives. Seriously, one of these babies kill 4 zerglings or one hydra alone no matter how well you micro. Being unarmored infantry, though, roaches are an excellent choice against them, especially with the burrow regeneration upgrade. Ultralisks can also take them down when in large groups, but they will be focused by the full army, and if the enemy has ranged units as well, it'll render them useless. Occasionally, protoss might try to rush you early with solely zealots, if you successfully scout this move, throw an extra queen into the mix, make some spine crawlers and banelings and cross your fingers.

Sentry: The ground support unit of the protoss. If you notice any of these near a choke, use air units or burrowed roaches to take them out before they reach the choke, as zerg rely on numbers, and horse fields can lock most of your units out of the fight. If you would end up having a portion of your army locked out, try to burrow all your units, as protoss relies on observers for detection and in low-tier matches (bronze to gold), they usually neglect making them.

Stalker: Stalkers are a lot like hydralisks, really, except instead of a range upgrade, they get to blink. In fights, they quickly eliminate your air support, therefore it's better to keep them at a distance unless you can sport superior numbers. When defending on high ground and on open ground, hydralisks will take them down pretty quick, the same thing works well when on low ground as well, however, you need to move in your air support or movable burrowing units in order to gain vision.

High Templar: Universal counter to zerg unless handled properly. As zerg rely on a large amount of relatively weak units, psionic storms alone can make you lose a 200-supply vs 200-supply fight. Handle them the same way you would handle sentries.

Dark Templar: Noobslaughter unit. Always have a few overseers scattered around your important places - choke, hatchery, main army.

Archon: Slow but powerful single-target melee attacker. Zerglings with attack speed upgrade or Hydralisks with range upgrade should get rid of them quickly, but don't let them get close to anything but your zerglings.

Immortal: Hard counter to ultralisks and resistant to all high damage mid-low attack speed units, such as hydras and brood lords. Zerglings with attack speed upgrade can swiftly take them out, but consider using neural parasite on them, as they can take down colossi and stalkers with relative ease.

Colossus: Big stompy laser beams which counter groups of ranged units, as all ranged units line up at their maximum firing range and colossi have the easiest time taking out units which are lined up nicely. They're glass cannons, so if you're only using ground units, you must focus them. Otherwise, mutalisks, corruptors and brood lords all counter them.

Phoenix: Their graviton beams are mostly useless against zerg, but their attacks will rape your brood lords. Corruptors in large groups and hydras in medium-sized groups will take them out easily.

Void Ray: They will mercilessly destroy all high health units, such as brood lords, ultralisks, hydralisks and corruptors. Your best bet is to swiftly take care of them with mutalisks.

Carrier: The unit which eliminates groups of zerg very fast when used properly. Corruptor focus fire should kill them before they can do significant damage, or if they are not available to you, focus your hydralisks on them. It's very important to kill them first as 8 interceptors potshotting you adds up quickly.

Mothership: Horribly useless unless you're out of overseers. You need to focus it, though, as if your opponent has one of this baby around, you can bet anything on him focusing your detectors first.

Protoss

Reserved.