Wiretap, by the Canadian Broadcasting Corporation
This is basically a 30-minute long program with readings of short fiction, and sketches usually recorded from telephone conversations (thus "wiretap"). It's really funny, particularly if you like This American Life. The host/narrator/producer is Jonathan Goldstein, who has worked a lot on This American Life.
A program of... basically news commentary by the two hosts, Seth Romatelli and Jonathan Larroquette, Uhh Yeah Dude is a humorous take on modern life and strange news stories. Usually over an hour long, pretty irreverent but extremely funny.
Just what it says, this is literally a massive historical survey of the history of Rome by Mike Duncan, from its legendary origins in the Aeneid and ending with the exile of the last emperor. There are five or six episodes dealing with Octavian Augustus alone. Each episode is about a half hour long.
howstuffworks.com actually has an entire family of informative and entertaining podcasts hosted on the site and through itunes. I mostly listen to Stuff You Missed in History Class hosted by Katie Lambert and Sarah Dowdey, Stuff You Should Know with Josh Clark and Charles "Chuck" Bryant, and Tech Stuff with Chris Pollette and Jonathan Strickland. Topics can vary from How Delta Force Works and Five Strange Government Experiments, to the history of the ill-fated Franklin Expedition and how RSS feeds work. Overall its really useful and has introduced me to a lot of new information.