I like Basic Brewing's podcasts a lot. On their website, you can go back in their archives and listen. You can download (for free) from itunes, also. I usually listen to them while I'm on the train.
Definately The Brewing Network with their 3 shows, The Sunday Session, Jamil Show ( older ones covered BJCP styles and the newer ones clone commercial beers ), and Brew Strong with Jamil Zainasheff and John Palmer.