The History of the English Language - This is very comprehensive and moves quite slowly forward through the history, with a lot of etymology along the way. It's only got up to Shakespeare and Elizabethan times just recently at around the 170th episode mark. So if you don't have an interest in language and linguistics then you might find it too slow, but I've enjoyed it.

Revolutions - Several series of episodes covering various revolutions (English, American, French, Russian, Mexican, etc). I most enjoyed listening to the French Revolution series (but haven't actually listened to them all). I knew the broad outlines of the French Revolution but had never properly 'studied' that period of history or read treatments of it so a lot of the detail was new to me. I actually had to stop for a bit and take a breather around the middle as the revolution just kept on going further and further to extremes and I was finding it just too crazy (not the podcast, the actual history).