Sandlot Science
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.