18.8 Riddles & Brainteasers
Two pages of links to Brainteasers.
From Thinks.com. The main page also has crosswords, word searches, and other fun things.
If you want to make yourself sick, this is probably the best one. It's from Ritsumei.
There are a lot of optical illusion sites on the web. There is lots of overlap between them. Here are just a few of the better ones:
Many more are easily found using a search engine.
Some artists have used optical illusions in their work. M.C. Escher is the most well-known.