10-15 minute videos for science
NOVA | scienceNOW | PBS

youtube has some great stuff.
eg 1: music videos


eg 2: lesson starters


http://streaming.discoveryeducation.com/
have some good videos, and broken down into small segments, but they require money for full downloads (thank-you firefix downloader ) and quite a few of the videos are a little oldish now.

i do not rate teachertube.com complete rubbish, but has potential one day