"The squares A and B on the illusion are of the same color (or shade), although they seem not to be. You can check it out with an eyedropper in a graphic program."
My favourite has to be the first one in this topic. I've seen it before but I still think it's great!
Your brain does an awful lot of processing on images before letting your consious mind see it. When it sees the picture it decides that it seems to be a regular patterned flat surface seen at an angle, with a shadow cast by a cylinder falling across it - none of this happens consiously. What is fed to your consious mind is a graphical description of a checkerboard pattern with a shadow. This pre-processsing of images is so strongly built in that the only way to stop it happening is to hide most of the clues that trigger it. If you cover the cylinder with one hand, and most of the checkerboard with the other - leaving just a thin slice with squares A and B in - you should be able to see them for what they are (but too much checkerboard and they still look different)!
My profile picture is one of those - although you can't see it happening with the image so small. Enlarged, the concentric circles are spinning.
This is what i find the best optical illusion. You can find here a lot of them here http://images.google.com/images?q=optical+illusions&hl=en&lr=&rls=GGLG,GGLG:2005-52,GGLG:en&sa=N&tab=ii&oi=imagest
Why don't you try this site for more mesmerizing stuff. http://www.ritsumei.ac.jp/~akitaoka/index-e.html