StarCraft/General strategies

Basic Zerg Strategy
The Zerg have some pretty awesome stuff going for them. To begin with, the Zerg have the unique ability to produce multiple units from only one building! This, combined with the fact that the Zerg's units are relatively cheap to produce, allows for masses of units to be quickly built. Therein lies the Zerg's true power, their units aren't very powerful, but they have a lot of them. Another interesting asset of the zerg is that all their units (including buildings) regenerate health over time, for free!

The Creep

Zerg have what is possibly the most restrictive building requirement in the game. The only buildings that don't require that they be built on the "creep" are the Hatchery and the Extractor. The creep is an expanding carpet that originates from Hatcheries; though it can be expanded by Creep Colonies. An important thing to remember is that any Zerg player may build upon any Creep. It doesn't have to be "your" creep. Indeed, players don't own the creep at all.

Upgrades

Upgrades are essential for the Zerg. Because the Zerg can produce so many units upgrading their attack and defense is very rewarding. Zerglings fully upgraded with Metabolic Boost, Adrenal Gland and Melee and Carapace upgrades can devastate an enemy's base before they know they are under attack. Burrowing is an extremely useful ability as it allows any ground unit aside from the Ultralisk to hide from enemy units. It also makes Zerg very effective at scouting as a burrowed unit can remain unobserved anywhere -unless a detector is nearby or a Terran player does a Scanner Sweep of the area- while still able to see all around.

Strategies
Six Pool Rush

Send your Drones to mine minerals immediately. Start Morphing one of your Larva into another Drone. Quickly send your Overlord to explore. Continue to mine and construct more Drones as soon as you have enough minerals. Once you have 6 Drones mining stop producing Drones. Continue to amass minerals until you have 200. The moment your minerals reach 200 produce a Spawning Pool. Build one more Drone while the Spawning Pool completes its morph and send it to mine. When your Spawning Pool completes you should have 3 Larva and at least 150 minerals. Use this to quickly produce six zerglings (two Zerglings hatch from each Egg). As soon as you have your Zerglings, attack your opponent with them. If you haven't already found your opponent with the Overlord the 'lings should find the base quickly. Note that the Six Pool Rush is most effective on smaller maps with few players. If executed properly you can usualy cripple if not outright kill your opponent. If the attack fails, however, your minutes are numbered as you will not have enough time to recover before your opponent finds and kills you. Weigh the ups and the downs of the Six Pool Rush carefully.

Four Pool Rush

This is essentially the same thing as the Six Pool Rush except that you build no Drones. Simply set your drones to mining and when you reach 200 morph one of them into a spawning pool. Don't build any more drones! When the pool completes you should either have 150 minerals or be extremely close to that total. Again, turn the three Larva into six Zerglings and rush their base. This is even faster then the Six Pool Rush but should not be attempted unless the map is very small.

Multiple Hatcheries

In most maps (and especially in "money maps") it is wise to have many Hatcheries so as to produce even more units even faster. If you have enough minerals also build extra Hatcheries at your expansions, this will allow you to rebuild quickly if your enemy finds a base.

Nydus Canal

Nydus Canals are buildings that allow ground units to move instantly from one end to another. After constructing the first end of a Nydus Canal you may place the other end on any creep that you can see. This allows you to move quickly from one base to the next so as to defend against attacks. Personally I've found the Nydus Canal to be instrumental in several instances where a Zerg opponent has guarded only the entrance to their base. Either infest a unit with Parasite (a Queen ability) or send a pack of Overlords or Scourge to your opponents base so that you can see their creep. Make sure that you have plenty of ground units and a Nydus Canal without an exit placed before you do this. Once you can see the area you are going to build on quickly place the exit of your Nydus Canal. Send all your units through the Canal and destroy as much of their base as possible before they notice you.

Basic Terran Strategies
Tank Push

Like the name, mass siege tanks and place them all near your enemy's base and put them in siege mode. The ones in front should fire at the base, and when more things go "boom", send the ones in the rear to the front and put them in siege mode and they should start laying waste. Continue until everything is destroyed.

Firebat Rush

This tactic is useful against the Protoss Zealots early in the game, but is extremely effective against the Zerg. Create the requisites for a Firebat quickly (Barracks, Refinery, Academy) and have at least 2 Barracks. Mass produce Firebats until you have 12 - 18 and rush the enemy base. This provides a viable alternative to other rushes in that the Firebats, with their 16 firepower, can also defend your base in the case that you get rushed. With their high firepower, cost-effectiveness, and spread damage, Firebats can easily decimate an enemy base.