Ok, these things are really neat. You need to look at the picture in a certain way, and it turns 3D. I imagine it like, if the screen is 2 feet in front of you, focus your eyes on what would be 4 feet from you. I really don't know how to explain how to look at these. It took me a while to see them for the first time. Some are neat and easy, others are stupid and hard. Have fun! Usually, the ones that look like static, or blurred set of tiny objects, are the best. Avoid the ones where it looks like a picture already.
P.S. You'll know when you're doing it right. Your eyes will be able to somewhat relax, and you see a clear 3D image. If you see some 3D stuff, but it looks kinda double-vision, blink a few times and look again.
Good ones:
http://www.eyetricks.com/3dstereo154.htm
http://www.eyetricks.com/3dstereo165.htm (http://www.eyetricks.com/3dstereo165.htm)
http://www.eyetricks.com/3dstereo166.htm (http://www.eyetricks.com/3dstereo166.htm)
http://www.eyetricks.com/3dstereo48.htm (http://www.eyetricks.com/3dstereo48.htm)
http://www.eyetricks.com/3dstereo95.htm (http://www.eyetricks.com/3dstereo95.htm)
Websites:
http://www.3dstereograms.com/
http://www.eyetricks.com/3dstereo.htm (http://www.eyetricks.com/3dstereo.htm)