A couple of weeks ago, I blogged about a screencast I did that showed how you could convert stereographs into red/blue 3D anaglyphs. Jim Gasperini has a nice writeup about “wiggle stereograms”, basically two frame animated gifs which toggle back and forth between two images.
On the right, you can see that I created one of these pretty quickly using the Gimp. Basically you take the left and right images, load them into two different layers, preview them using the Filter>Animation>Playback tool, and then save the result as a gif, and check the box that makes you save them as an animation.
Kind of neat!