Lienzo is a Video Annotation tool focused on letting users express their creativity in a collaborative way enhancing the media experience. It allows you to simply create rich annotations or sketches over videos and share it with others.
You can think of lienzo as a sum of youtube with a paintbrush, mixed together in a seamless experience
The source code is not available at the moment but you can give it a try at:
Lienzo – A collaborative video annotation web based tool