So, I recently inherited an iPod touch from my brother. Being the "sketchy" fellow that I am, I promptly downloaded a very wonderful free app. It is called "SketchbookX". It is a very fun app to play with. Whenever you get the itch to sketch, this app will do you good.
So far I've only tried the free version. I have Sketchbook Pro for my computer and it is a very full software. I am sure that I've only scratched the surface.