Anamorphic Illusions

You've probably seen this before. Julian Beever.
http://users.skynet.be/J.Beever/pave.htm

for example, view this from right side up.
http://users.skynet.be/J.Beever/globe.html

then view the same pic from upside down.
http://users.skynet.be/J.Beever/globe-wrongview.html

My favourite one is the batman one
http://users.skynet.be/J.Beever/batman.html