This might be old, but it’s new to me: The Checker Board Optical Illusion. Take a look at it and try it out. You’ll click the solution 10 times and block out half the image with your hand and still not believe it. J-Walk did some cutting and pasting and proved that square A and B are indeed the same colour. I did the same. They’re the same colour. It’s freaky. (Here’s another proof.)
While I’m at it, here’s another one. First, just look at. Then shut one eye and go in close and concentrate on the cross in the middle (any slight eye movement can ruin the illusion). Within seconds, all you see is the fuzzy green dot going around in circles. The other dots disappear. Open both your eyes and move back from the monitor. Pretty neat.
(via J-Walk’s blog)