Someone recommended
In Our Time to me a few months ago and I've been really loving it. Not really a podcast proper, just a podcast of a BBC Radio show. Each episode is about a topic (really anything and everything; history, science, culture, etc.) and features the regular host as well as three guest academics who know a lot about the topic.
As well as being able to get it on iTunes, there's a laughably massive archive on the website; seriously, just search a topic you're interested in and they'll have probably done an episode on it.
http://www.bbc.co.uk/radio4/features/in-our-time/