I walk to and from work, so I listen to a LOT of podcasts. Some of my favourites:
Anything BBC (Friday night comedy, History Magazine, Adam and Joe, anything I can find on history or art...)
Anything NPR (Wait Wait Don't Tell Me, Car Talk, This American Life, Driveway Moments for sure)
I also have a bunch of random ones from "Itunes U" which are lectures on tons of different subjects (so I have some on ancient history and some on physics).
I LOVE podcasts!