I bought Plastic too and liked their system. Some others I have are below. Ink animator is to expensive to me but appears to be a rising star. I use Pro animation now ($9.99). There is also Doink, but I don't think it has good export features.
https://itunes.apple.com/us/app/animation-pro/id1098505288?mt=8
https://itunes.apple.com/us/app/ink-animator/id1056941883?mt=8
https://itunes.apple.com/us/app/animation-drawing-by-do-ink/id364762290?mt=8