You *have to* keep an eye on Google's TechTalks. I have not seen an obvious link, so here it is a search and a feed on that search (better, since it provides longer descriptions). They are mostly about computer science, but the range of topics is quite broad.
It is great that they are choosing to publish all these terrific talks they host.
Update: from http://video.google.com/googleplex.html
Google TechTalks are designed to disseminate a wide spectrum of views on topics ranging from Current Affairs, Science, Engineering, Humanities, Business, Law, Entertainment, Medicine, and the Arts.I also learned about the Authors@Google series:
Authors@Google is a speaker series where thought-provoking, Zeitgeist-making, trend-setting authors come to the Googleplex to read from their works and share their thoughts with us. The following authors have agreed to release their talks to the world on Google Video.