Completely and totally not even necessary.
Nobody needs this, but it's so much fun, we had to bring it back.
BackLight is an all new System Preference Panel, which gives you the option to place a quick-access menu item (NSStatusItem) on your menu bar. This allows you to turn your screensaver on and off as your desktop background, on a whim.
Backlight can also change your screensaver and open the preference pane.
Simply choose the "Start Screen Effects" menu item to start running a screen effect as the your background. Then choose "Stop Screen Effects" to stop it and return to your normal background picture. Doing so will start the screen effect that is checked in the "Screen Effects" menu.
To change which module is running, simply select the desired screen effect from the "Screen Effects" menu. Once an effect has been chosen, it will start running as your background and is set to be the screen effect that is run when the "Start Screen Effects" menu item is selected.
This concept originally appeared on NeXTSTEP as "BackSpace", and then on Mac OS X as BackLight, from Freshly Squeezed Software. The illumineX BackLight is a System Preference Pane.