I came across the Koffka ring and the Checkershadow optical illusions earlier this morning. Very cool! Here’s the checkershadow in-lined:
(the square labelled ‘A’ and the square labelled ‘B’ are the same color!)
Seeing these two optical illusions reminded me of the rotating snakes optical illusions that I think were slashdotted a long time ago. He’s got a few new ones up that are as interesting and along similar lines as the original.