I don't really review iPhone apps, because I don't have an iPhone and I think it is important to test an app on its target device, but Shapesynth is deserving of an honorary "Awesome!" rating. It sounds like a Nintendo, while looking like an Atari, but it has so many features! Today's update improves upon the "Slave algorithm", which allows for wirelessly networked MIDI control from its Brother app.
Shapesynth is also the first music app to support the Sphero, a ball-shaped accelerometer controller. This works like a cheaper ($129) version of Percussa's Audiocubes, which are so expensive they won't even tell you the price.... I seem to remember them being around $700 for 4 (only available in sets).
Here is a cool demo of Sphero controlling the FM in Shapesynth: