Star6 is a newly released remixing musical "instrument" for the iPhone, and also has one of the best GUIs I've seen on an iPhone app. The application lets you create music and sounds by mixing your own samples, or using the ones provided. You can chop up beats, pitch samples up and down, alter the speed, gate, sample order, sample size, and randomness. Furthermore, all of these features can be tweaked by tilting the iPhone in various ways.
Everything can be recorded and wirelessly synced with a computer through WiFi. Take a look at this video, it'll show you everything the app can do:
If you're a musician, or just someone who likes playing around with beats and samples, then this app is a must. Pick it up in the App Store for $6.99. For more details, go here.