One of the features that piqued the interest of quite a few people when iPhone OS 3.0 was originally announced was Apple finally allowing developers access to the iPhone/iPod touch music library. Gamers were anxious to see something similar to Audiosurf, a PC game that turns music tracks in to actual in-game tracks for you to race on. Unfortunately, the limited access developers actually ended up with to the device's music library prevented much more than the in-game music player menus that have been implemented in some games.
Avatar Labs' recently released Rhythm Racer [App Store] doesn't allow you to play your own tracks, but it does a good job of providing a similar Audiosurf-like experience. You race down a track, tilting to move your ship right or left to follow the notes and fly over jumps to collect bonus rings.
The controls work well, and the music combines with the gameplay to create a really neat experience, especially if you've never played Audiosurf. Rhythm Racer is even free to try, initially loaded with a single track. Within the game, you can buy a second track for 99¢, but that's all the extra content that seems to be available currently.
The sensation of speed is good, and while there isn't much to do in the game after you race through the two tracks, the game uses OpenFeint for both online leaderboards and achievements which could provide some replay value if competing on online laderboards is your thing. Since the game with one track is free, it's definitely worth a download, if for no reason other than to hold you over until Riddim Ribbon arrives.
App Store Link: Rhythm Racer, Free (DLC Track: 99¢)