Coincidentally (or not?), both inkycircus and LabLit have articles discussing and pointing to a bunch of different sources of science-related video content on the web.
Personally, I love this kind of stuff and could watch it all day if I let myself. A few of the suggestions from the two articles, featuring both new shows/videos and oldies-but-goodies:
- Lab Reporter: "films to take you into the heart of science"
- Secret Life of Machines
- 2006 Year in Videos from New Scientist
Of course, searches on YouTube and Google Video also turn up a lot of interesting content. Enjoy!