Animoog can play as a controller as well, with the paid midi out upgrade. RSTouch is simply amazing, a musthave if you own a pad and reason. Other than that theres seriously way too many apps to list. Try poking around the app store a bit, never know what you might find in there.
MS20 mini, XStation, Monotribe, MalletKAT,
Reason, Recycle, Reaper, Tassman, Chromaphone,
Radical Piano, ReTron, Antidote, Polar, PolySix, Buffre.