If you're like me, you're constantly changing your background, perusing through a folder filled with potential wallpapers and then selecting a new one only to find, an hour later, that you want to change it again. (Or maybe I'm just a bit too into customization!)
For customizers like myself, there's some great free software called John's Background Switcher, which will automatically change your wallpaper on whatever schedule you desire (yes, even every hour!).
You can choose to have the pictures come from a folder on your computer like "My Pictures," a set of individual photos, or even from the web via photo-sharing sites like flickr, Phanfare, smugmug, picasa, or even Yahoo image search!
Of course, you can also fine-tune the settings as much as you want - specifying whether images should be full-screened, tiles, montages, always in sephia tones, etc. If there's a setting you don't see, the software's creator even invites you to tell me what you want via his contact form.
The software is available for free (donations accepted) and works on all versions of Windows - 93-Vista (but did not state if it works on 64-bit or not). The download link is here.
(image via John's site)