1. Hardcore History [0]. Even if you're not a fan of history, check it out because Dan really grips you with the story. I can't say enough good things about this podcast and look forward to each new episode. The episodes are LONG; they're more like audiobooks in a way. The series on World War I (Blueprint for Armageddon) [1] was six episodes and 24 hours long. Perfect for road trips, but the downside is there's only 2-4 a year.
2. The British History Podcast [2]. Fills the gaps between Hardcore History releases. Jamie does a weekly, very deep dive into the whole of British history, starting from prehistory up through (currently) Alfred the Great. He's a bit more funny and sarcastic than Dan is, but you still end up learning a great deal.
3. Escape Pod [3]. Science fiction short stories covering a wide range within the genre.
[0] http://www.dancarlin.com/hardcore-history-series/
[1] http://www.dancarlin.com/product/hardcore-history-50-bluepri...