Well, we've seen many variations of this illusion, but it's good enough to be worthy of another incarnation, and this one is interactive. If you click on the image you will be taken to a java applet where you can vary the various features of the display to your heart's content, making the illusion as strong or as weak as possible for your specific sensorium. What's the illusion, by the way? The two gray circles are the same shade of gray, despite appearances.