Audiosurf - Very cool little game for $10. I heard of it when it was highlighted as one of the PAX 10 indie games this year. Basically, it's cross between Tetris and a racing game: You're piloting a vehicle along a roller-coaster-like track, and hitting blocks to form color-matching sets. There are a bunch of different modes, in three rough groupings of difficulty, including one popular one where you simply try to hit colored blocks, while avoiding gray ones.
But the cool thing about the game is that the shape of the track, and the color and density of blocks, are calculated from some kind of waveform analysis of your own CDs or MP3s, which then play while you run that track. For example:
It should be obvious that one of the joys of this game is plugging obscure, weird, eclectic songs into it to see what it does with them. Each song has its own individual online leaderboard, so you can compare your performance with others. For example, here's an obscure song (from the A Mighty Wind soundtrack album), but not so obscure that I'm the only one ever to play it, where I currently hold the top spot in Pro mode.