Here’s a big page-full of well-presented optical illusions and paradoxes. Many of these are oldies-but-goodies, but there are more than a few that were new to me. Many of these are startling. The site offers succinct explanations for many of the illusions.
Related post:http://dangerousintersection.org/2006/12/08/optical-illusion-extravaganza/
And don’t forget that we are subject to mental illusions too.