This game has been around for a bit on a few different platforms, but just thought I'd share that it's free on iPhone/iPad atm and works surprisingly well on it.
If you don't know of it, it's a twinstick shooter like Geometry Wars that uses your music library for not only the songs, but enemy spawns/spawn rate, weapon strength, speed of enemies, basically everything having to do with the gameplay. One of the power ups is volume up even, so it's very centered on music.
It's also on 360, I think PS3, and on Steam which is the better option of the three with a free demo you can try it out with.
Oh, as a general warning, it can get VERY flashy if you're prone to seisures =P