These experimental games from GDC sound incredibly cool. They use sound input, either from a music file or live from the player, to generate and control the game. I'm especially intrigued by this:
I once thought it would be cool to make a rail shooter that worked in a similar way, but where the environment was generated from the wave--bass sounds might create the ground landscape, while treble would create enemies or obstacles. Barret's take is more interesting, because it lets the player trigger the music from a landscape generated from the sound file, thus giving the player a real investment (similar to the incentive of finishing a Guitar Hero song, not because of the score, but because you want it to sound good).