This catched my eye on steamfriends Mario Kart Source. It looks fun, but it seems like a straight copy from the Nintendo versions.
Article: http://www.steamfriends.com/mods/p2_articleid/617
movie: http://www.youtube.com/watch?v=XEzu1SLKB2c&eurl=http://www.steamfriends.com/mods/p2_articleid/617
screenshots:
Article: http://www.steamfriends.com/mods/p2_articleid/617
movie: http://www.youtube.com/watch?v=XEzu1SLKB2c&eurl=http://www.steamfriends.com/mods/p2_articleid/617
screenshots: