There are some really cool illusions available in the latest theme pack for Windows 7

Download Illusions theme

Optical illusions, planets of grass floating in impossible summer skies, and homages to M.C. Escher fill this free Windows 7 theme of weird and wonderful digital art by Josh Sommers.”

download illusion theme by Josh Summers

Download Illusions theme

Update 02/09/2011 just a quick shout out to jennifer who has posted some more information about this theme and Josh Sommers on the official Windows team Blog

Rob