Here’s a large collection of illusions of all types. It’s fun to peruse them and see how wrong our initial perceptions can be.
I already wrote about this illusion, but it’s mentioned again and uses a different approach to show that the squares are the same color. It’s hard to convince people that their brain is that wrong.
I especially liked motion induced blindness, sigma motion and the face in the beans (it took me a while to see it).
(Note: Most of the illusions use Flash, some require animated GIFs, and a few are quicktime movies)