The Gifmelter takes an animated GIF file and can, through the powers of dark magic, transform them into interactive images. Imagine that "moving" part of the file as a paintbrush, and you're on the right track.
It works for any GIF - just bookmark Gifmelter and click on it when you've got an image open - but it really takes off when some thought goes into the source material, and you're using a "clipped" file instead of just a framed image/animation. Like this, for example.
It's the work of Tim Baker and Chris Shier, and you can check it out - along with some examples - below.