Motion Induced Blindness: In this striking optical illusion, staring at the central blinking green dot will cause the three stationary yellow dots to intermittently disappear. At first, I wondered if I was being fooled by the animation software and that the yellow dots really were disappearing. But of course every time I looked directly at the yellow dots they never wavered; they only disappeared when I looked at the green dot.

The Nature paper on this effect is available here (.pdf format), but the scientific basis of this illusion is still not well-understood. More interesting optical illusions are available here. (Via Linkfilter.)