Thumbjam by Sonosaurus.
Thumbjam is an iPad/iPhone app that comes with tons of very high quality samples built in. It lets you easily perform parts, record them as loops and play them all on the iPad or iPhone.
This is the best sampler/jamming tool I have ever used. In addition to making it easy to put together jams on the iPad, you can use ThumbJam as a wireless controller for your desktop DAW.
With Logic 9, for example, via some free middleware (DSMidiWifi), you can play your solos on the iPad and directly record the midi onto any track in Logic 9. This gives me countless guitar and amp variations as well as any other instrument.
It is also somewhat easy to create your own samples to download to the iPad to extend ThumbJams when you are away from your desktop. When running solo on the iPad, however, it is a bit tricky to get your recorded loops to all synchronize but with practice it works pretty well.
I put together a small blues piece with Logic tracks and 2 solo Thumbjam tracks for the guitar and horn section.
This will definitely be part of my future music making tools and is one of my favorite iPad apps to-date.