Developer website: Joakim Nygård
Under MacOS 9 you could control-click an image file and use it as a desktop image. Under MacOS X you have to go to the System Prefs, click Desktop Pictures and then drag (or otherwise select) the file you want on your desktop. I got tired of this. Drag an image file onto DesktopControl (currently supports jpg, gif, png and tiff) and your desktop will instantly (well, almost) change.
Mac OS X 10.1 or later