http://allfinearts.com/wp-content/uploads/2007/08/illusion-small.jpg

I saw that and thats what kinda.. Inspired, if you will, to make this thread.

EDIT: Heres another one, kinda hurts your eyes:
http://www.graphicsdb.com/data/media/580/3D%20Waves%20Optical%20Illusion.jpg

EDIT EDIT: Yet another one:
http://www.optillusions.com/dp/1-41.htm

I've gotten really good at that last one from playing Brain Age.