Quartz Composer is a visual programming environment, included with Xcode, that allows you to quickly create sophisticated motion graphics compositions without having to write code.
The interaction design community's interest was piqued recently when Julie Zhou mentioned Quartz Composer was popular with Facebook's team.
The official description above fails to mention a key strength of the tool though: the easy ability to make your compositions interactive and responsive to keyboard, mouse or multitouch inputs.
I'd been on the lookout for a tool that could go beyond Keynote's broadly default iOS-equivalent interactions palette when Quartz Composer started getting coverage, so I decided to jump in and give it a go.
The best place to get started is with Dave O'Brien's excellent tutorials. There is also an excellent branch thread and burgeoning forum.
Dave O'Brien has started this with some very useful helper patches.
I've also generalised my first composition, details to follow...