Here is a searchable database of educational videos from a variety of sources (Youtube, readwritethink, National Geographic, eHow, etc.) Most clips are embedded on the WatchKnow site, but some are linked to the originating URL instead. Viewers can search, rate and comment on content. (The quality of videos ranges from "home made" teacher-produced videos to more polished "professional" clips.)
"WatchKnow is a project of the non-profit Community Foundation of Northwest Mississippi, and is directed by Dr. Larry Sanger, best known as co-founder of Wikipedia, and edited by a group of editors and assistants, all trained as educators. WatchKnow plans to index over 50,000 educational videos by the end of 2010. Imagine, in the long run, cataloguing and creating millions of short videos, so that one can quickly and easily find a top-quality video on absolutely anything a child is studying."