WallShuffle allows you to regularly change your wallpaper. You select a folder, then select a duration to apply a random picture as a desktop background. You can start the program in minimized mode with the "-m" argument, if you don't want to see it, or even hide the icon from the tray and forget it.
AutoWallpaper is a cross-platform desktop wallpaper changing utility. It automatically (or manually) changes the desktop wallpaper at a specified time interval set by the user. It runs in the background with a system tray icon that is visible when the application is running. It is written in Visual C# .NET 2008 (Windows versions) and Qt 4.4.3/4.5.0 (Linux/others).