One of the most amazing illusions I’ve ever seen, The Eclipse of Mars Illusion (Hat Tip: Mark). Follow the instructions on the page and take the time to do it. You’ll be amazed at the results. The side effects are pretty cool too.
When I did it I saw it rotate around the circle clockwise. thelsdj in #swhack said it was most likely do to my eyes wandering and my dominant eye wanting to go clockwise. Makes sense. Let me know if you see it rotate. Also, the side effect after doing it was that I saw a blueish tinted circle whenever I looked at my monitor. You did you see afterwards?
The Eclipse of Mars Illusion is my favourite illusion, but there are meny more at Mighty Optical Illusions. Check them out.