If you stare at this image for 20-30 seconds, it should fade away until it completely disappears. I found that it is not quite that simple, but I eventually got it to work.
This is called the Troxler effect, after the man who discovered it; what happens is that your visual system constantly adapts to all the external stimuli. This is why after spending a few moments in the dark, you start to see a little better. This capability allows you to be in different lighting conditions, while still maintaining a pretty accurate estimate of the lightness and colour of objects.
The caveat is that you must keep your gaze completely still. The slightest flick of the eye will make the colors come back. It may help if you place your cursor on the image and focus on the cursor to keep your eye in place. This particular image works because there are no lines or angles to draw your attention. It might also help to close one eye. -via Boing Boing