I've been interested in history for a long time. So as I commute to work from the Cleveland area, I've been listening to lectures by Yale professor Paul H. Freedman from his course, "The Early Middle Ages, 284-1000."
Freedman generously allowed his course to be placed on the Internet as part of Yale University's Open Yale Courses program, which in turn is part of the 450 free online college courses compiled at Open Culture.
Some of the classes, such as Freedman's, are available as both video and audio. Some are offered only as streaming video.
Course areas include architecture, the world of the ancient Greek and Roman classics, economics, film, geography, law, literature, astronomy, biology, physics ... you get the idea. There's a wide range of courses.
I'll probably listen to lectures on quantum mechanics and ancient Greece when I finish Professor Freedman's course. The ancient Greece lectures are by Donald Kagan, a big name classical historian.