I enjoy the How Stuff Works
group of podcasts, as someone earlier in the thread mentioned. There are about 10 different ones to choose from, on a variety of subjects. I prefer Stuff You Missed in History Class and Stuff Mom Never Told You. They have about two episodes a week, but they are usually only about 30 minutes long and very self-contained to one specific topic, so I often skip the episodes on topics I am not that interested in.