csGrain is an audio processing powerhouse with 10 effects to morph your samples or live audio input. There are a lot of options for getting your samples into the app; AudioPaste, Dropbox and even iTunes songs can be imported. I think the best use is as an effects box for guitars or other instruments, especially if you have an interface for audio input. The sample import can be fun if you want to make ambient drones or scary remixes. The available effects are Pitch Shifting, Ring Mod, Chorus, Flanger, Delay, Reverse, High/Low Pass Filters, and Reverb. Additionally there is a compressor available in live audio mode. Due to the heavy processing involved, an iPad 2 or greater is required. I have a guide for installing csGrain on iPad 1s, but this is unsupported and performance will be slightly limited.
Disclosure: I received my copy for review from Boulanger Labs.