I can't believe no one has mentioned Adobe Photoshop Touch. It is incredible. I have been using Photoshop on PC and Mac for many years and I can assure you that PS Touch comes very close to the real thing. It is an amazing piece of coding. If you are serious about digital imaging, this is a must have.
I won't consider using PS Touch until they increase the output resolution. 1600 x 1200 is unacceptable. I'm sure a lot of photographers agree.