MadPad which is one of the apps from the people at Smule. It comes with some saved sets, but the cool thing is that with an iPad2 you can record video into each of 12 boxes. Then you use your fingers to play the madpad like a drum or piano. When you press the box of the video that sound will play. Check out this video.
This opens up lots of possibilities for composition. I have several students working on madPad for our concert next week. I like how many of the iPad apps have exporting functions that make it easy to use. I would love if there was a notation function (as odd as that sounds...).
This is me. First try.