Audiosurf is a neat little music/racing game. You select a song from your collection, and Audiosurf builds a track and sets the tempo based on the music. The goal is to run over colored blocks on the track and rack up a big score.
This is a Steam-based game so it also tracks how other people have done on the same song (though my musical tastes are obscure enough that no one else had played with any of the tracks I used), and there are medals and achivements to be won.
For only $9.95, this is an excellent casual game.
