This is a project I've been working on in the past weeks: http://www.spaculator.com/

It's a silly app where people guess the daily bitcoin price of a chosen day and put a small amount of bitcoin to support their guess. The closer the guess, the bigger the reward. And there is no username/password required to login; instead, only a bitcoin address is required.

So yeah, you spend bitcoin to speculate on bitcoin and maybe get more bitcoin out of it. Feedback appreciated.