The Amazing Dots
Watch the X in the middle very closely. You should start to see a green dot that rotates around the circle - this dot is an illusion; then you should see see the purple dots disappear.... but they haven't really gone. It is an after image effect, sometimes called a 'negative retinal afterimage' - move your head slightly, and the dots will reappear... amazing or what?
This illusion was designed by Jeremy L Hinton of Bristol, UK, and this illusion - with a lot more explanation and many other illusions as well - can be found at the web site of Michael Bach.
If you liked this illusion, have a look at the Hollow Face Illusion...