Here's a fun little flickr gallery of "transparent desktop screens". Users take a picture of whatever lies directly behind their displays, crop them accordingly, set the picture as their new wallpaper, and take a picture of the whole set up. Ultimately a lot of time spent on something that only works if you don't move the computer at all, it's a novel little visual trick nonetheless (and not Photoshopped, either.) If nothing else, it might give you some inspiration for fun desktop wallpapers of your own. Or you could use someone else's and pretend to be on another desk altogether.