UC Berkeley, Stanford, MIT, Harvard, Yale and many other schools have partnered with YouTube to make an official channel under the banner YouTube EDU.
There is interesting content from classrooms or lecture halls with subject from almost all the fields of science. It is a great opportunity for learning or reviewing material of high quality.
As I mentioned in a previous post, MIT has launched the MIT OpenCourseWare. Many of the school courses are available free online.
Berkeley also has launched a site with video and audio webcasts of classes.
If you have some free time to spend then free higher learning is a great investment.