magic eye tetris - it'll hurt your eyes!
 

If you've never looked through magic eye books then your shit out of luck. JUST KIDDING! Here's the link: http://www.lutanho.net/play/magiceyetetris.html If you've never done these before, you have to look past your monitor until you see the image in 3D, it shouldn't look blurry when you have it right. After playing this for 5 minutes my eyes started hurting.

There's also a magic eye quake 2 mod here: http://www.leweyg.com/download/SIRD/q2/index.html looks interesting but I don't have quake 2.