I used to have a whole bunch on transparencies that I would put on the overhead projector and show my students on the last day of school. They're so fun and freaky (the illusions, not the students...well on second thought...). Anyway, I saw this one today on Boing Boing. Squares A and B below are the same shade of gray.
My first thought was NO WAY! But then I isolated the two squares in question, and sure enough--they're the same color!