iGendyn (App Store link) is a moderately interesting synthesis app that’s let’s you play with Xenakis’ general dynamic stochastic synthesis.
It’s essentially a powerful noisemaker. You can make some pretty rough-edged sounds with this; the iPhone gives you surprisingly sensitive control surface. The accelerometer-based controls are less sensitive and harder to make reproduceable gestures with; the (up to) triple-touch control is easier to learn and control.
It might be useful for a situation where you need its swoops and slides; the performance gestures make it something that might have a good bit of interest onstage; it feels a bit like you’re wrestling with the phone when you’re really going for it and attempting to control the sounds at the same time.
It’s free, and definitely worth a try for something different.