I'm a fan of visual illusions, but this one is by far the most impressive one that I've ever seen.
It shows an image of a castle with inverted colours. When you move the mouse over the image it is replaced with a black and white version of the same image. Stare at the image for a few seconds, and then, without breaking your stare, move your mouse over the image.....
http://www.johnsadowski.com/big_spanish_castle.html
Unbelievable!
EDIT: Oh, and this
isn't one of those screaming zombie pranks. Promise!