ChangeDesktop is an application that will change your Desktop picture at random from any number of folders you choose. It is written in Cocoa, completely free and the source code is available.

Features:
-Changes your Desktop picture at a specified interval on up to eight monitors.
-Installs a system wide hot key so you change your Desktop picture from any application.
-Can choose a picture at random, or choose each picture in sequence. If pictures are chosen at random, ChangeDesktop can be configured to use all pictures before repeating any picture.
-Coalesces pictures from multiple folders so you don't have to move all of your pictures to one folder. Sub-folders are also searched.
-Will notice additions and removals to any folder without having to restart the application.
-iTunes integration allows desktop changes to be synced with iTunes track changes.
-If your system goes to sleep, the ChangeDesktop Server can be told to change the desktop picture when the system wakes up.
-Thumbnail pictures are automatically filtered so they will not be displayed. This is particularly useful with iPhoto.
-Web Pics feature that allows you get pictures from any webserver and automatically add them to your collection. Includes support for Rendezvous (Jaguar and later) and the capability to archive the downloaded pictures.
-Remote Access feature that will allow anyone to view your current desktop picture from any web browser. Also included are the currently playing iTunes song and system statistics such as processor speed, memory usage, and uptime. Remote Access also has the ability to take screenshots of your Desktop.
-Publish Desktop feature that allows the Server to automatically upload your desktop picture to any FTP server every time the desktop is changed.
-Includes a Menu Extra that allows you to launch ChangeDesktop or change the Desktop picture.
-Supports MacOS aliases.
-Fully compatible with Panther's Fast User Switching.
-Localized for French.

Requirements: OS X 10.1 or higher with the BSD subsystem installed