I mentioned the amazing painting tool "Colors" earlier.
I recently stumbled upon an animation software called "Animanatee". It's simple, superfast and very user friendly. It supports even multiple undo and one onionskin layer. Perfect for straight forward animation in the subway. ^^
There is another animation software called "FlickBook" but I didn't like the user interface. It was a bit too complicated but it has more functions though.
Anyways both have even an avi file exporter! How cool is that!? Here is a lil' walkcycle I animated with animanatee. If you own a DS....Animanatee is a must have! ^^v