I listen to a bunch of podcasts, the history of rome has stopped, but is still so worth listening to it still deserves to be mentioned, the others are still active. Sorted by category:
gaming: the escapist podcasts,
history: the history of rome, revolutions, dan carlins hardcore history (very long episodes, ideal for transatlantic flights..)
comedy:mybrothermybrotherandme, the bugle, caustic soda (very gory, each week they take a subject and what can go horrebly wrong with it), the complete guide to everything, judge john hodgeman
science:the escapist science podcast, the skeptics guide to the universe, startalk, the naked scientist, science friday, skeptoid.
not fitting other categories: radiolab, tedtalks ,freakonomics, sawbones.