What's cooler than listening to your music while playing a video game? Having your music actually become the game! AudioSurf uses the data in your music files (it supports most common formats, such as MP3, M4A, and Ogg) to create a sort of "racetrack". You pilot one of many different ships (only a handful are unlocked in the demo, and the demo only allows five plays) along this curving track, scooping up colored squares. As you do so, patterns form, and whenever you get three--or more--to touch, they 'pop' (similar to Tetris).
There's quite a bit of strategy here--'hot colors', such as red and yellow, are worth more points, so you want to grab them--but 'cool' colors are much more common. If you try to overfill a column, it goes away, costing you points. The various 'characters' have special powers, such as picking up blocks you can deposit in a better place, and there's "specials" and "power ups" to go for, as well.
The coolest part, though, is that the track, the pacing of the game, and the placement of blocks is all dynamically generated from your music. This is not the first game to use data files on your disk as a means of generating a game environment, but it's one of the most fun I've seen to use that technique.
There's an online community component as well, so you can compare your score on a given song to that of people near you, or all over the world. There's even an online "radio station" which plays songs if you don't have any of your own that you want to use.
The game is installed and managed using Steam, a tool for delivering--and controlling--digital content, used by many gaming companies. If you don't have it installed, the download link will do it for you. You cannot download the game file alone. Steam is spyware-free and reputable, but it does communicate data from your computer to its servers, so you cannot play this game without Internet access even if you do not wish to engage in online competition.