I really liked the plug-in system from gedit, but it is licensed under the GPL. It would also require a lot of changes to be re-usable outside of gedit. I should know, thats what I did in my syndication reader prototype, Marina. The framework is simplistic and has been incredibly successful at converting users into application extenders. If GNOME desktop applications can provide a single framework for extending applications, the barrier to entry becomes much lower for "repeat extenders". We instantly re-purpose those application scripters into new applications.
git clone git://git.dronelabs.com/ethos