Think Klax + Amplitude. Audiosurf is an independent game by Invisible Handlebar that generates its levels entirely from music on your computer. You load the game up, select one of the 14 game modes (Three major game modes, eleven variations of them to make them more challenging), select a song and start playing.
The object of the game is to control your vehicle as it rides along a multi-laned track while colliding with colored blocks to form chains of at least three of the same color. The speed and shape of the stage, the amount of blocks in it, and their positioning is all determined by the music file you specify. Of course, the background music for the level is also the specified track.
You accumulate points by racking up huge chains of the same color (Some colors are worth more than others) or hitting special blocks that, when dropped to the bottom of your playing area, instantly give you points. Completing other tasks, such as avoiding any grey blocks in one type of game mode or getting almost all of a certain color in another, will net you bonuses at the end of the song. The game then uploads your scores and people who play the same song can match their best runs against yours.
Here's a couple of videos to show what the gameplay's like:
The game runs for $10 on Steam. I strongly recommend dropping the money on it as it's an incredibly awesome use of a few minutes if you've got the free time every now and then.
Jam it back in, in the dark.