UC Berkeley has been posting podcasts of classes and events for quite some time, and you can listen for free. Offerings range from the history of American Cyberculture to Organic Chemistry. They also have several appearances from Michael Pollan, including talks on his books The Omnivore's Dilemma and The Past, Present, and Future of Food. In addition to his solo events, he also moderates a panel on The Politics of Obesity.
You'll need RealPlayer (the free download at http://www.real.com works fine) to play the webcasts.