I was bored one day, so I threw together a really basic mountain range (which I then textured to look like green glass) using Bryce 5.5 and then fucked with the sky until it looked weird and green. It took me about 5 or 10 minutes to throw it together, and I decided that I liked it, so it's been my background for the last year or two. I use it on both of my computers (although for a while I was using a streetlamp that I made, also in Bryce, until the imperfections bothered me so much that I had to either fix it or change to a different background).
At the moment, I'm feeling too lazy and tired to post a picture of it (it's 3:00 AM, I really need to go to bed), but I may be bothered to put it up sooner or later.