I’m a big fan of The Teaching Company – they seek out some of the best professors in the nation and publish top-notch lectures. Unfortunately, their video and audio courses can be quite expensive (i.e. several hundred dollars for a course series).
Recently, The Teaching Company has been adding mini-lectures to YouTube. Although you do have to listen to a brief ad about purchasing the full course, these lectures are a useful find if you have a few spare minutes.
Currently, the mini-lectures available are: Game Theory, the History of Christian Theology, Cosmology, Chaos, and Building Great Sentences.
Another tip: Sign up for a free account on The Teaching Company website and free, full-length audio lectures will show up on your account page. You can stream or download these lectures in mp3 format. If you live in a major city, your account will also list educational / cultural events in your area. There is no cost for either of these lifelong learning services.