My wife and I are both big fans of the college courses produced by the Teaching Company. The courses cover a wide variety of subjects, and come in a range of video and/or audio formats.
I personally find that the audio format usually works somewhat better. The lecturers are very good, but watching a professor lecture on TV is inevitably somewhat dull. On the other hand, when I’m driving or riding in a car or train, I find that an audio lecture fills an ideal amount of mental bandwidth. (Every so often, the lecture gets complicated at the same time as the driving, but one can always rewind).
Otherwise, there are unfortunately not that many science and mathematics courses that go beyond the beginning undergraduate level, although I did enjoy Stephen Nowicki’s course on Biology. If you are interested in history or philosophy, or other subjects in the humanities (like my wife, who is a historian) there are many more interesting options.
The Teaching Company has an unusual pricing policy. The courses are very expensive, except when they go on sale, when they cost roughly one quarter the normal price and are very reasonable. All the courses go on sale on a regular rotation, so unless you are in a tremendous hurry, you should definitely wait until the course you are interested in goes on sale. A lot of the courses are available at libraries too, so you can borrow one to see if you like it first.
Here’s another blog post endorsing the Teaching Company, with some reader comments on their favorite courses.