I’ve been a fan of the Teaching Company for quite a while. Their entire business consists of finding the best college professors in the world, then convincing them to record lectures on a wide variety of topics, which they then distribute for very reasonable prices. Most of their content is available in DRM-free MP3 form, and I regularly listen to lectures while doing housework and other chores - a very good use of time.
While their business selection is a little sparse, the Teaching Company really excels in “liberal arts” courses, particularly history, philosophy, and physical science. Every single program I’ve purchased from them has been superb - the professors are engaging, the content is well-structured, and the presentation is superb. Listening to one of these lectures is like attending a high-level Ivy League college course in the subject without the hassle of homework, exams, or six-figure tuition bills.