I listen to 'Stuff You Should Know' by Chuck and Josh at HowStuffWorks.com. The topic is usually very interesting and the hosts are very entertaining. While it is usually very well researched, I like that it is light enough that I don't have to pay attention to every sentence.

Who Charted and Doug Loves Movies are pretty good 'brain off' entertainment as well. And I've listened to Planet Money and This American Life for years and years and years.