Via BoingBoing (again), here's an amazingly trippy visual illusion I've never seen before. Stare at the negative image of the castle for twenty or thirty seconds then, while staring at the black dot in the middle, move the mouse over the pic. The photo appears to be in color until you move your eyes, when it becomes clear that it's actually a black and white photo.
This one feels like a magic trick; the color literally vanishes right before your eyes! But, to paraphrase Doug Henning, "it's only an illusion."