my favorite app is Phyle's Wallpaper Switcher.
i hate staring at the same background day after day, and have at least 130 wallpapers on my hard drive, various pictures i enjoy
but, changing through them is a pain, of course, so i found this:
http://www.totalshareware.com/asp/detail_view.asp?application=16038
install, set your image directory, and it will randomly change your wallpaper in time increments, from one minute, to startup only, to every 6 hours or so
i get ALOT of wallpapers from:
www.digitalblasphemy.com
havent subscribed yet, always free previews there, and lots of great stuff in the users gallery, the users stuff usually has links to their page, with even more great wallpapers in alot of cases
oh, and terragen is sweet for people who wanna mess around with terrains, can do some high quality stuff
its free, if you register you get expanded features, very easy to use, and has an enormous amount of scenery variables
www.planetside.co.uk/terragen/