It's like playing a game of tetris, yet you use your own music, and instead of blocks the blocks are notes, its definitely an interesting game to play with and see what you can do.
Extract from the website:
Ride your music.
Audiosurf is a music-adapting puzzle racer where you use your own music to create your own experience. The shape, the speed, and the mood of each ride is determined by the song you choose. You earn points for clustering together blocks of the same color on the highway, and compete with others on the internet for the high score on your favorite songs.
Audiosurf on Steam includes The Orange Box soundtrack, integrated with the game to enable"Still Alive"surfing and more. In addition, Audiosurf is one of the first titles to leverage the recently announced Steamworks, offering full support for the Steam Achievements that appear on Steam Community profile pages.
Play with any music from your own collection--the game supports MP3s, iTunes M4As, WMAs, CDs and OGGs
Valve's The Orange Box Original Soundtrack is included.
Online scoreboards for each song you play.
Choose your strategy from 14 available characters.
Track your progress as Steam Achievements.
Let me know if you have played this game as well?