"We've introduced a shopping cart to Steam. To celebrate AudioSurf is now just $2.49, this week only. ADD TO CART!" implores the Steam homepage.
AudioSurf takes users' music in a variety of file formats, including MP3 and audio CD, and creates a racetrack based on the song. Racers then collect blocks in sync with the song's rhythm. At that price, it's well worth trying out.