Here is another great free Windows 7 theme by the digital artist Josh Sommers that provides a more complete introduction to the breath of this creative style. Illusions theme brings planets of grass floating in impossible summer skies, optical illusions, and homages to M.C. Escher. This is a really intriguing, weird and wonderful set of wallpapers; 19 in total.
Check this article from the Windows Experience Blog to find out more about the artist.