The History of Rome - does what it says on the tin. If you are interested in that sort of thing, bingo. Interesting, amusing, thorough and fascinating. And nearly 200 episodes long, so if you're into the whole brevity thing, it ain't for you.
The Butterfly Effect - Jon Ronsons' podcast on the death of a porn star and the murky world of adult entertainment. Always an entertaining author he translates well to podcasts.
Adam Buxton - remember Adam and Joe on c4 in the late 90's? Yeah, it's him whilst Joe is off being a very successful director in that there Hollywood.
I'll be gutted when I've finished them