My senior year in high school I got hooked on an online multiplayer Tetris game called TetriNET. I eventually created a pretty sophisticated bot to play for me, which absolutely crushed the games. A friend recommended I submit it for consideration in the regional science fair, where I wound up taking first in the computer science category.

Lots of details on the blog [1], including screenshots, source code, and details of the algorithms used.

[1] http://www.mattmazur.com/2009/05/creating-a-tetrinet-bot/