This app goes in the "I can't believe you can do this on a iPad" category. Animation Desk (U.S. $4.99) lets you create freehand animations using a generous set of drawing tools and brushes. If you have any artistic ability at all, you can create animations that look professional. The interface Animation Desk presents looks similar to the tools an animator would use in the physical world. Make your drawing, add a new page and redraw with the changes. You can make really good looking animations for business presentations, for animated e-cards for friends, or just for your own enjoyment. When you're done, you can upload to YouTube or share your work on Facebook.
Other nice features are the ability to add sound effects, and there are multiple frame rates, from 3 FPS to 24 FPS.