I have used this site for two graduate seminar classes that I got credit for. I had to write up a curriculum proposal, and write reviews on all the ones i watched for the class.

http://videocast.nih.gov/default.asp

It has alot of health talks but also has many off topic science talks including talks from the witnesses in the Dover case and Medical studies looking at the effect of prayer (there was no effect, suprise)

I liked the evolution of different diseases talks.

Hope this site isn't a repost and have fun looking through thousands of vids.