The animated image below consists of 12 lilac, blurred disks arranged in a circle 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.
Stare at the "+" in the center of the array for 20 seconds or so.
Pretty cool optical illusion.
For an explaination, go here: http://en.wikipedia.org/wiki/Lilac_chaser
Stare at the "+" in the center of the array for 20 seconds or so.

Pretty cool optical illusion.
For an explaination, go here: http://en.wikipedia.org/wiki/Lilac_chaser