This is a good one:
Stare at the center dot and watch what happens.
Found at
BrainBashers.com. They have a lot of illusions, some of them already linked to ITT, but they have a mouse-over Javascript doohickey that isolates what you're looking at from what's causing the illusion. So you don't need to open MS Paint to prove that those chessboard squares were the same color.