If you want to learn about video signals and programming microcontrollers this is a pretty good project, i've seen it in kitset form at the local electronics store also.
The guy implements a Pong and Tetris game that plugs into the TV with about $5 of electronics:
Pic Pong
The guy implements a Pong and Tetris game that plugs into the TV with about $5 of electronics:
Pic Pong