This site is called "fun things" and it has lots of illusions and other experiments to illustrate how your eye works. If you've never seen you blind spot (where the optic nerve enters the retina) be sure to try that link!
This site is a treasure trove of optical illusions! Some of them require a Shockwave or Java plug in.
If you're curious about vision correction without eyeglasses try
This is just the beginning- the web has thousands of sites devoted to optics, eyesight and illusions. If you find something really great, let me know!