Another really fun site is Optical Illusions, etc. http://illusionsetc.blogspot.com/
It's a blog that collects all types of illusions, so the content changes every couple of days. I particular the host seems to like (as I do) illusions that occur spontaneously : Look for the Pope or Devil Illusion about a third of the way down the page.
"Ars longa, vita brevis." (Life is short, the art so long to learn) -Hippocrates