Audiosurf must be one the coolest things I've ever seen.
It's a cross between listening to music, a racing game, Tetris and the winamp visualizer.
You basically race a hovercraft thing along a track whilst picking up/avoiding different coloured blocks. The track however is formed by loading an MP3 of your choice into the program. Hard to explain, so just look at some of the videos:
http://www.youtube.com/results?search_q ... arch_type=