http://www.patmedia.net/marklevinson/cool/cool_illusion.html
Thanks for giving me a headache. That is pretty cool though, even when I llooked away there was a 'trail' of green dots.
Maybe not a green "dot" per se, but there's definitely something green in there... Image Unavailable, Please Login
The cones in your retina are responsible for color. If you over tax them, the chemical reaction that triggers the electrical impulse to the brain runs out of chemical and the opposite color shows up. Similar to the effect you see if you look at a red object intently for 30 seconds, close your eyes and you see it in the opposite color (greenish).
Now I see why that cat keeps you around! There are a bunch of interesting illusions here: 59 Optical Illusions & Visual Phenomena