[Note: this strategy will only really work if your base has only one avenue of attack. Also it works best against zerg but will also work fairly well against everyone else]

The ultmiate test (so far) of my strategy was this: myself playing Zerg alone against 5 computer players all playing Zerg.

Send your ovelord scouting (duh).
Make a drone.
Make a drone.
Make a drone.
Make a spawning pool.
Make a drone.
Make an overlord.
Make a drone.
Make an extractor.
(The cool part here is that the pool and the overlord will hatch just as you reach 150 minerals and get your 3rd larva. Another strength is that if the computer is rushing you then your zerglings will hatch just before the attack comes in)
As soon as the pool hatches make 6 zerglings.
As soon as the extractor hatches set 3 drones mining it.

Send your zerglings to go looking for trouble.
Keep making them zerglings and keep putting them in the same squad as the first 6.
As soon as you have 100 gas give the zerglings the speed upgrade.
As soon as you reach 100 gas again upgrade your hatchery.

Once your zerglings have encountered an enemy base tell them to attack the nearest building. Under Broodwars AI all the drones within will swarm out to attack you. Run away.

You heard me run the he|| away! Run to a point roughly halfway between your base and their's. At this point you will almost outnumber the drones that have still been blindly following you. Wipe them out, all of them.

Then run back to their base and kill any drones that are left. While this is going on keep making zerglings but also try and get a den and the lurker upgrade. The zerglings that are made from this point on should be used to look for the second enemy base. When they find it do as you did before: attack and then retreat, bringing the drones with you. Take them to a point roughly halfway between this second squad and the first squad.

Meanwhile back at the enemy base all of the drones are dead. Run away.

You heard me: run away. Once all of your opponent's drones are dead then they are dead too because they have no way of building more drones and thus no way of doing ANYTHING for the rest of the game. Now go and do the same to the 2nd enemy base and the 3rd.

By now you should have enough lurkers to defend you base from pretty much anything they can throw at you in the early game.

When you rush the 3rd base you will encounter creep colonies. Ignore them. Attack the nearest building and get the drones to follow you out (be mindful of the fact that with the speed upgrade your zerglings will leave the drones behind if you aren't careful). Slaughter them. Rush back in and kill any that are left. Taking the pain from the creep colonies sucks but it is do-able.

Right! Now you have incapacitated 3 of your enemies. This means that it is now you against 2 computers and any moron can win that.

The real name of this strategy is '4R': Rush, Retreat, Reinforce, Reattack.

So far it is the only method I know of beating 5 computer players. If any one knows of any others I will be impressed.

CreepColony's Opinion: A good strategy for defeating comps early in the game, but if you're using zerglings, they might outrun the drones, so don't run too fast.