buildzion.com

Nature (the popular PBS series)
http://www.pbs.org/wnet/nature/category/video/watch-full-episodes
Nova
http://www.hulu.com/nova
Scientific American
http://www.hulu.com/scientific-american-frontiers
Discovery - Science
http://science.discovery.com/videos/
Discovery - Education
http://streaming.discoveryeducation.com/
Academic Earth (College Lectures)
http://www.academicearth.org

http://video.nationalgeographic.com/video/