An actual business strategy that is proven to work, and if done right is completely legal. Consists of the first person signing up, who then gets X friends to sign up, and those get X friends to sign up, and can give something away for "free" (eg. freeipods.com). This works on the principal that even if EVERY person in America was to sign up, the bottom of the pyramid (which would consist of most likely more than half the US population) wouldn't be able to get the X friends and couldn't complete the offer. The scheme will make them money no matter what happens, even in the worst case scenarios.
Aww man, I tried to sign up for freeipods.com, but all my friends already have an account!