LivingSocial offers daily deals. Buy the deal. Share the deal (via Facebook, Twitter, etc.). If three people go for the deal after clicking through your shared link, you get it for free.

Woot buys something in bulk (electronics, tools, home goods, you name it) and sells only that for one day at a good price. Then the next day (or hour, if a Woot off is going on), they sell something new.