Great wallpaper gives your computer an ever-changing appearance that breaks up the monotony of looking at the same old boring screen all the time. One of the first essentials of a wallpaper collection is a program to change the images at a desired frequency.....
WallpaperChanger (freeware)
Wallpaper Sequencer (free trial, has some nice 'extras')
WallMaster (freeware)
Wallpaper Xe (free trial)
Now, for some wallpaper sources:
Digital Blasphemy (Computer generated images)
NOAA photo library (Over 20,000 weather-related images, all free)
Hubble Space Telescope (Awesome images of our universe)
Caedes Desktop Wallpaper and Artist Community (Submissions from members. A collection of anything and everything.)
.... and plenty more available using your favorite search tool.