Sprite Something Sprite Something allows you to easily make pixel art, and then animate it. It's much easier to use than something like Photoshop
Zen Brush Zen Brush is the first drawing app that I used where it felt like I was creating just as smoothy as on paper. It's really nice for creating backgrounds.
GarageBand I was skeptical when they said Garageband on the iPad could be useful for composers, but it really is. Sure it's very stripped down, but it's great for throwing down quick ideas.
Thicket Thickit is a strange little music app. Every gesture has it's own little sound and by combining them you can create some really crazy effects. It's more of a toy, but it's free.
SoundPrism Pro Soundprism Pro is quite expensive for an app, but really worth it. Because of it's unique layout it is really easy to create harmonies by synching it up to your computer and using it as a control device.