In my SketchUp enrichment class yesterday, I was showing the kids some of the illusion models I made in SketchUp a few years ago, for my Optical Illusions book. I hadn't look at these in a while, and it was fun to see the kids' reactions.
This one can make you a little dizzy:
The blurry diamonds are made using different shades of gray and turning off edges.
You can download this model from the 3D Warehouse. (The Warehouse has lots of illusion models; if you want to find more of mine, search for "roskes illusion.")
Here's another favorite - stare at the black dot and move your head toward and away from the screen, to "see" the trapezoids spin around.
Download this one as well.
There are many more illusion models, including ones that play with color and parallel lines. Have fun!
Anyone can design anything in 3D! http://www.3dvinci.net/