Here's mine. You ever play Erotic Photo Hunt? It's a game on those touchscreen video kiosks in bars. Each round, you're shown two almost-identical pictures of a scantily-clad girl. You've got a minute to find the 5 subtle differences in the two photos. (It's embarrassingly addictive -- for both men and women alike.)

Build a web-based version of that. After every round, the player is given two options... "play the next round" or "see more pictures of this girl." The latter is a paid link. Weekend hack => goldmine.