Just saw a TV commercial for Crayola 3D sidewalk chalk. Kids were drawing on the sidewalk with this chalk, and when they put on special glasses they saw it in 3D. Needless to say the televised 3D effects were simulated, so I dug around to see if there really is a 3D effect.
It's the real deal. The glasses are the important piece, not the chalk. The glasses in the Crayola kit utilize ChromaDepth, a patented diffraction effect that makes red look closer, blue furthest, and colors in between (on the spectrum) fall in the middle. Reading reviews, the chalk in the Crayola kit is probably more colorful than typical sidewalk chalk, but it's the glasses that are the important bit.
Chromatek's most recent patent (from 1991) has some good drawings that show how ChromaDepth works.
Chromatek sells glasses directly, a set of three for $7 + s/h. I'm almost tempted to buy a set just to see it in action.