Podcasts are great. For those not aware, all Podcasts on iTunes are free. What a deal...
In no particular order, here's some of the ones I've been listening to:
The Moth - Really engaging real life stories told on stage in front of a live audience; great - 10-20 minutes in length
Dan Carlin's Hardcore History - Theater of the Mind-type history lessons, really, really good stuff. This guy will draw you in. The 3 part series on the Punic Wars was amazing. 60-90 minutes each.
Tank Riot - 3 guys from Wisconsin who like to pick a subject and talk about it, could be anything from recumbant bikes to beer to what an asshole Thomas Edison was. They miss the mark sometimes, but grow on you. The episode on the life of Mister Rogers brought tears to my eyes. But I'll never admit it in person...
This American Life - The well known NPR show about an hour
Stuff You Should Know - The whole 'Stuff' series is pretty good; 20 minutes
Point of Inquiry - I don't subject the guys to this one. Interviews with scientists, educators, authors, etc.
Podcast Alley is a good resource for finding interesting stuff