I love optical illusions. I had a friend once who insisted on calling them optical "delusions." In any case, I wanted to post a site called Mighty Optical Illusions  that has the best optical illusions I have ever seen. A new one is posted just about every day, and the site has a great Random Illusion feature if you just want to sit and scroll through dozens of them and try to figure them out.

Below left is an example of an "impossible structure." On the right is "The Haunted Forest." How many different creatures can you find in the forest?

The site offers widgets that you can download to get the Optical Illusion of the Day delivered right to your desktop. I am running the Vista widget on my computer at home.