You may have noticed a friend or a coworker with a fun new sci-fi background theme for windows from GT Graphics and thought “why can’t I have that on my non-windows computer?”. You can! Just go click around from http://www.gtgraphics.de for an hour or so (to say nothing of dealing with the .rar files). :(
Or you can run this script to download all the backgrounds into folders based on image size :)
Note that you’ll need the PHP rar extension to extract the older artwork. Also, some of his rar archives are, sadly, broken.