Quartz Composer 3.0 in Leopard contains templates for Graphic Animation (Keynote backgrounds, etc); Graphic Transition (iMovie '08 transitions); Image Filter (iChat effects); Music Visualizer (iTunes visualizers); RSS Visualizer (?); and Screen Savers. Create a new composition from one of these templates, put your composition into /Library/Compositions (or ~/Library/Compositions if you're timid) and it will be available to any application that requests a list of that class of composition.

I've been playing with the iTunes visualizers and iChat effects. I put a few in the "Compositions" page. This sure makes it easy to extend the built-in effects; I'm amazed more online publications (especially Ars Technica!) haven't expounded on this feature at length. Video effects for the masses! Viva la revolution!