I love a good background on my desktops - it just makes everything look better. I just found another good source for images: InterfaceLIFT. Unfortunately, like many free wallpaper sites, the pages are laden with ads; fortunately, the images are well worth the visual cacophony.
If you are wanting to see what Vista (the next version of Windows) may use for it's default backgrounds, look no futher than this random page.
Visual appetite not yet sated? Check out Michael Swanson's blog for some great macro photography of nature.