I am constantly amazed by StereoGrams.

Directions: Either look to infinity and then slowly bring your eyes back to normal convergence while looking at this, or cross your eyes, and then slowly bring then back to normal. If you find the right eye alignment, you will see a 3-d image appear above the screen.

Here are a few I have found: 

bird.jpg (456093 bytes) Bird        Aircar.jpg (530177 bytes) Air Car      indysys.jpg (355527 bytes) indysys

mdino.jpg (370725 bytes) MDino   warper.jpg (580805 bytes) Warper      vortexas2.gif (463431 bytes) Vortex

rid_n.gif (112799 bytes) Riddle  rid_t.gif (100539 bytes)Riddle  horsecolor.jpg (298903 bytes) Horse
 Pumpkin   Teapot  Hearts
akutsu.jpg (38943 bytes) Shuttle  starwars.jpg (350589 bytes) Star Wars  Spiral
3D    747   Coin
Cosmos  Stars   Dolphins
Wolf   Cube   Knot
Excellent

 

StereoGram Riddle
StereoGram Riddle II