My alma mater, UC Berkeley, had just put over 300 hours of videos on YouTube covering lectures on biology, physics, chemistry, and so on. It says that it's the first university to offer full courses available on YouTube.
http://www.youtube.com/ucberkeley | Here's a good one: Google co-founder Sergey Brin's lecture on search engines - via Ars Technica
They're organized by semester. Enjoy
http://webcast.berkeley.edu/courses.php