In Our Time is a fantastic source of interesting topics, especially since all it's past episodes are available for free on the BBC's website. With over 640 episodes it shouldn't be too hard to find something of interest!
For those not familiar with the show, the format is simple. A topic is picked for each week and it consists of the host, Melvyn Bragg, and three experts on that topic. The experts are then left to discuss the topic for 45 minutes with the host only interjecting to ask questions and steer the conversation. It's very much the pinnacle of listening to smart people talk about things that they're interested in.
Recent episodes have covered topics as diverse as The Talmud, Photosynthesis, States of Matter, Spartacus and Early Chinese History.
A list of episiodes can be found here - http://www.bbc.co.uk/programmes/b006qykl/episodes/player - and there's links on the site to download all the episodes since 2011.