Some time ago I came across a great desktop background website, InterfaceLift.com. When I first went to it, there weren’t that many images, but the number of background images they have seems to be steadily growing.
I decided it would be nice to get a copy of all the images for myself so I built a little scraper to do the job for me. I wrote it ruby so that I could take advantage of a great ruby gem called Watir. The script works by going through all desktop backgrounds, selecting the largest available and making a nice link in a separate text file. Afterwards a download manager like DownThemAll can be used to download all of them.
At the moment the script works in Windows with Internet Explorer but I am working on making it work with FireFox 3 in Linux. Its wiki page is here.