Seriously.js is seriously cool (heh) client-side realtime compositor for video and images by Brian Chirls.
The demo video, courtesy of Ok Go, provides a fun showcase of its abilities (gotta love the nightvision/Modern Warfare effect!) and demonstrates the realtime transitions made possible by a node-based compositing approach.
Having not played with this kind of library before, I was pleasantly surprised how quick and easy it is to pull something together. As usual, the demo source is worth a poke around