Click on the image and stare at the center cross for at least 30 seconds to experience the three phenomena of the illusion.

Lilac chaser is a visual illusion, also known as the Pac-Man illusion. It consists of 12 lilac (or pink, rose or magenta), blurred disks arranged in a circle (like the numbers on a clock), around a small black, central cross on a grey background. One of the disks disappears briefly (for about 0.1 second), then the next (about 0.125 second later), and the next, and so on, in a clockwise direction. When one stares at the cross for about 20 seconds or so, one first sees a gap running around the circle of lilac disks, then a green disk running around the circle of lilac disks, then a green disk running around on the grey background, the lilac disks having disappeared or having been erased by the green disk. This is due to an interesting effect in which the colors of the lilac disks are inverted in the optical illusion to a green color.
From Wikipedia.

For more optical illusions, have a look at colourlovers.