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.