http://dto.github.com/notebook/remix-this-game.html
Basically DTO has written a neat game that I demo'd before but you can edit just about everything inside the game. The assets (music, audio, pictures, levels) and even the source code.
He's got binaries for many platforms or you can just download the code from github.
He also writes: http://lispgamesdev.blogspot.com/
http://dto.github.com/notebook/xong.html
abram