One of the best examples of optical illusions, if not the best, I have come across is the one shown below, the Checker Shadow Illusion by Edward Adelson, professor of Vision Science at the dept. of Brain and Cognitive Sciences at the Massachusetts Institute of Technology.
The squares marked A and B appear to be two different shades of gray. In fact, both squares have exactly the same shade of gray, but most viewers will refuse to accept this. When you move your mouse pointer over the image you can eliminate the signals from the environment that are send to your brain, and you'll see the true colors. In case you're still not convinced, or suspect faul play, you can download the image, and use the PhotoShop eyedropper tool to determine the RGB values for these squares.
Have a look at these explanatory pages at the MIT website to find out how and why this illusion works. At the same website you'll find more interesting optical illusions. Other Internet sites on the subject that merit your visit are the Grand Illusions and MindFake websites.