This is the Grand daddy of all optical illusion sites. Some of the illusions are interactive using Java, and some tell you how to create your own illusions. Illusionworks Another site with optical illusions. On this site, there are more mathematical explanations of what is happening.
Some illusions are interactive, but require shockwave plug in.
Optical Illusions Choose Optical Illusions on the home page to view. You need to scroll down to find it. All the usual optical illusions plus a few new ones. A fun site to visit.