Tonight I played AudioSurf. It's a music/rhythm game where you move your ship left and right on the track to collect blocks and avoid obstacles. The big twist here is that the tracks are procedurally generated from any music you want! You can select tracks from your own library and play them.
There is an online leader board, so you can compete with friends and strangers to get high scores on any track. I really enjoy playing this game, it can be really relaxing!