The Planet Perplex Website at http://www.planetperplex.com/en/index.html.
The Might Illusions Blog at http://www.moillusions.com/ has postings going back to February of 2006.
Many people love Michael Bach's site at http://www.michaelbach.de/ot/, especially the following eyes.
The Exploratorium in San Francisco has their illusion exhibit at
Please leave your favorite illusion website in the comments section below.