National Geographic offers some stunning, professional-quality wallpaper photos, but only as individual downloads. If you're a frequent wallpaper switcher, or like background variety, a simple command line script makes it easy to grab them all.
In Linux (and, presumably, Mac's terminal), the two scripts offered by the Web Upd8 are easy enough to run. Download either natgeo2008.sh for National Geographic's 2008 photography contest selections, or natgeo2009.sh for 2009. Make the script able to run by entering chmod a+x natgeo2009.sh (or substitute for 2008), then move it to a folder where you want the downloads to appear. Run the script (./natgeo2009.sh or something similar, in most cases), and you'll get your downloads—226 for 2008, 170 in 2009.
The 2008 wallpapers have variable resolutions you can grab, while 2009 comes in at 1600 pixels wide. For details on customising the scripts to grab different resolutions, hit the Web Upd8 link. Windows users can still pull off this script by installing and setting up Cygwin.