In Our Time is an excellent weekly radio program from the BBC. The really cool aspect is that the beeb is making it available as a podcast (here's the RSS feed).
The host invites well known scientists, philosophers, historians, etc. to debate and discuss a central question or theme. Recently we've had topics such as prime numbers, Geoffrey Chaucer, human evolution, and Catherine the Great. Fascinating stuff.
Warning: only the latest podcast is available, and then only for seven days. So make sure you run your podcast client regularly!