Audiosurf is a hybrid of rythm/puzzle game for the PC plataform. The concept is fairly simple: You rush through a "roller coaster"-like course controlling a ship (simillar to the ones from the F-Zero series), and through the course you must collect colored blocks in order to form clusters, and thus earn points depending on how big of a cluster you form, and the color of the blocks. There are plenty of variations to this mechanic for you to try (all in the form of differenct characters, divided in three levels of difficulty: Easy, Normal and Hard).
But the true beauty of the game lies in the fact that the course through which you'll run is entirely generated based on the music of your choice (the game accept most of the more widely used formats, like .mp3, .ogg, .wav, etc...), so that th experience is all up to you and your musical tastes. If you want a fast, blood fevering experience, try some fast music with lots of beats. If you want a more relaxing experience, try riding on some calm or sweet melody.
The game also features a robust online leaderboard system, showing the high-scores for each individual music you play, separated by difficulty level. You can check the global scores, or check the nearer scores (based on your location), or even check your friends scores (by adding other players to your friend's list). You can also view the most popular songs, and also the most recent musics your friends have been playing. The game's overall experience is highly enjoyable, and it is an entirely new way to appreciate your music collection.
(For those who already know it and play it, my username is FoxDeimos, you may add me as a friend anytime

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http://www.audio-surf.com/ <- The game's main site, with screens, videos and downloads.