In Ken Perlin’s recent DrawPad project, he explores creating an animation through sketching. There is a nice, simple visualization of history, which enables you to see your animation by rolling over it, edit the animation and insert new sketches.
It was developed with pen computing in mind and therefore uses a number of gesteral commands, so definitely check out the user’s manual. The working demo is a lot of fun. The linking/zooming feature is particularly cool, underscoring the goal of informal communication.