I came across an awesome optical illusion on Digg. Stare at the center of the image for 15-30 seconds, and then move your mouse over it. You’ll see a color image, but it’s not really there. Move your eyes away and back to see how it really looks.
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ChipLog 
I think it is based on complementary colors: they “taint” your retina (persistence of vision) by showing you see the “negative” on the image first so your retina adapts and tends to equalize all areas of the image, then you see the B&W version of the positive image, and for a while your brain continues to equalize and you see the positive image in color. Very nice. It reminds me of another illusion, which I believe relies on the same principle:
http://www.psy.ritsumei.ac.jp/~akitaoka/zanzoe.html
And another one that I cannot find right now, with rotating purple dots that appear green (the negative of purple) after a while.
This page is the best source I know for AWESOME illusions:
http://www.ritsumei.ac.jp/~akitaoka/index-e.html