Stereograms
These are some of the better ones I have found on the web
Stag and fawn
H2O Molecules
Hexagon
Picture of a Virus
Maelstrom
Plus
Propeller
Intertwined
Rings and Cube
Helicopter
Smooth Spiral
Starfish
Dripping Tap
Teapot
Vortex
Whirlpool
Cone