The AVA (Advanced Video Annotations) project is a video annotation service that is suitable for research, teaching or learning. Architected as a stand-alone service, AVA can be integrated with most video players, video management systems, learning management systems, and lecture capture systems.
An implementation is done with
a) SWITCHcast: http://www.switch.ch/cast
b) Opencast Matterhorn: http://opencast.org/matterhorn/