In our time with Melvyn Bragg is a wonderful podcast. i cannot recommend it highly enough. He is this brilliant guy who just talks about the most fascinating things ever with the world's leading experts on those things.
I just listened to his podcast on the oracle at delphi, the new one is about unicorns, also available for free - imaginary numbers, the spanish armada, german thinkers of the 1800s, it's so great.
Please listen to it.
Also: archives available for online listening here: http://www.bbc.co.uk/radio4/features/in-our-time/archive/a/all