Probably the app I use the most. They recently added some features and functionality that has helped to get some better results out of your drawing. Definitely worth picking up if you're an artist, but I would suggest getting some kind of pen with a rubber tip to use with it.
A new one that I picked up the other day. Its a fun and highly functional little animation program. I wouldn't use it for anything finished, but if I just want to rough out an animation while sitting on the couch, its a great starting point. I plan on figuring out a way to export it as frames to I can do the final clean up, ink, and paint on the computer.
While I still plan on making my game some day, I found that this app helps me sit around and create assets while I've got the TV on. I basically export it to the Photo section on my iPad and then email the pictures to myself. I'll have to composite it into a spritesheet when I'm done.