The active ingredient is XUL, a markup language (the eXtensible [or "XML-Based"] User-interface Language, to be precise) that describes things like toolbars, menus, keyboard shortcuts.
Just was curious to know how many of the Digg users use the Swarm/Stack features. It was the much hyped once upon a time but I just viewed the swarn/stack a couple of times just to see how does it work. Please provide your vote. Thanks!