Propellerhead have come out with a great little iPhone app for making some quick tunes on the fly. The app is called Figure and features a simple but beautiful interface that includes some instruments found in Propellerhead's music software Reason.
The app is very intuitive and you can lay down drum, bass and lead tracks pretty quickly with the end result being not that bad. The only downside is that the app doesn't allow you to save or export your creation, well that's actually a big thing. Until they remedy that, Figure is still a lot of fun to play with.