This is a pretty cool site showing various optical illusions. It does a pretty good job of both demonstating the illusions while telling their history as well as showing why they occur. There are 67 illusions along with dozens of variations. I enjoy the fact that someone has gone to the trouble of putting all of these in one place. Although I agree with one comment on digg about needing some tylenol after viewing too many of them. And of course, this is another case of someone having LOTS of time on their hands, but worth checking out.