Just search for "optical illusions" - there are hundreds out there, but these are by the same author as the one above - I have actually seen them before, but the one above was emailed to me:

http://web.mit.edu/persci/gaz/gaz-teaching/index.html

This site seems to have quite a few varied illusions:

http://www.skytopia.com/project/illusion/illusion.html

Gareth