I've been playing wound with Exposure on iOS and I have to say it's capable of kicking out some pretty impressive images. Take the two below as examples - I spent just a couple of minutes on each and I'm sure people with more patience and know-how will be able to do much, much better stuff.
Not too bad at all, really.
Exposure can manipulate specific areas of a photo in all kinds of weird and wonderful ways. I've always fancied doing things like selective colour removal but never had a tool or the knowledge to do it. Exposure makes it simple and it's super cheap, too.