A variation of the Hollow Face Illusion, the Gathering for Gardner Dragon's head will appear to follow the viewer around up, down, right, and left as he moves. Actually it's just a paper model, and there' not a little motor in the neck.
Not technically an illusion, but I found solids of constant width to be interesting. Basically they're oddly-shaped solids with a constant width (duh) that allows them to behave like spheres. In this video, a person puts a book flat on top of 3 solids and can roll the book in any direction smoothly, something you'd think could only be achieved with spheres. Among other things, these geometrical shapes allow you to drill square holes.