MR1200 - like an SL1200, but for MP3s
Excellent - a simple, functional DJ-style MP3 player. Not hardware, unfortunately, but unlike the other apps I've seen, it doesn't try to do the world, just what a turntable would do.
Very cool feature: vinyl groove simulation, so you can see major track features like you can on a record. This is big; being able to see the quiet bits on a record is really helpful.