Wallperizer is free software that lets you change the background of your computer automatically in a short amount of time.
This is a very lightweight (410 kb), portable (which does not need to be installed) that works in Windows Xp, Vista, 7.
Once downloaded, unpacked and started, simply select the folders on our computer where the wallpaper to use and simple to set the parameters to load. You can add up to 100 directories where the program would look for pictures which would be used as wallpapers.
Wallpaperizer also comes with a calendar, which can be displayed on the wallpaper. Calendars have skins. You can edit, load and save calendar skins. See this example.
If you are using Linux, try a similar application called Desktop Drapes or maybe the wonderful Dynamic wallpaper which automatically changes your desktop background based on based on the weather, moon phase, time of day, etc.