If you haven't checked out TEDTalks check the link and/or pull it up on iTunes. There are literally dozens of some of the most interesting speakers doing short (less than 30 mins) talks on a topic. Some are funny, others are serious, but all of them are thoughtful, interesting and enlightening.
Some of my favorites include Ze Frank, Richard Dawkins, Steven Pinker, Daniel Dennet, Micheal Shermer, Julia Sweeney, Steven Levitt.