Les Jones

Dancing Baloney > The Checker-shadow Illusion

Found at Willisms, who in turn found it at Marginal Revolutions.

I really couldn't believe squares A and B were the same color. Will convinced himself of it by using Photoshop's eyedropper to sample the colors. I decided to remove the squares from the surrounding picture. Here they are with the adjacent squares:

They still looked like different colors to me, so I took out the surrounding squares. Now I can finally see that they're the same color. That is one heck of an illusion.