Quartz Composer files can be used as backdrops in iChat and Photo Booth. This means some screen savers can be used as backdrops!
In the Finder, use the Spotlight search box to find files ending with .qtz. Then in either iChat or Photo Booth, open the Video Effects section, navigate to the User Backdrops section, then simply drag a .qtz file into one of the unused backdrops. Finally, click the backdrop you just added to see the effect!