General Science Video Lectures
These are general sites that offer hundreds of video and audio lectures in a wide variety of subjects. Many of the sites overlap a great deal in content, but there are also some courses that are unique to one site or another, so its worth checking them all out and search each for whatever topic you are interested in.
Specific University Free Lectures
These are links to the websites where some major universities have made a number of their lecture series freely available online. A mixture of audio and video content, sometimes also with access to course notes and other materials.
- UC Berkeley: http://webcast.berkeley.edu/series#c,s
- MIT Open Courseware: http://ocw.mit.edu/courses/
- Open Yale: http://oyc.yale.edu/courses
- UC SanDiego: http://podcast.ucsd.edu/
- UCLA Bruincast: http://www.bruincast.ucla.edu/