Compiz Cube Background
I really like how the Compiz Desktop Cube makes the monitor feel like it can do more than show a 2D desktop. To enhance this effect, I used the Skydome part of Compiz and turned Animate Skydome on.
Here is the picture that I made for the background. It uses the preinstalled blue marble background that comes with Ubuntu 10.04. Just point your file browser to that file and it will use it as the background.
Update this thread with good looking backgrounds that stretch what people think of the desktop environment.