I used to subscribe to In Our Time (I especially loved the episode on the Scientific Cavendishes), but it was getting too hit-and-miss for me, and the host started getting on my nerves a bit.
BBC 4 podcasts are great, though. I still listen to Thinking Allowed, Friday Night Comedy (love love love News Quiz!!!) and Thought for the Day (I'm an atheist, but will take my thoughts wherever I can find them!).