I'm a lecture junkie. I have previously blogged about Little Gray Books, one of my favorite online lecture series. A new resource for finding online lectures has come to my attention: Listening to Words. The site contains a database of lectures (990 of them), which can be searched, browsed by lecturer, or by lecture location. This is a gold mine of spoken word content, people! Get downloading!
Best bets: The Fog of War: Robert S. McNamara and Errol Morris in Discussion, Bill Bryson at The Royal Society on "A Short History of Nearly Everything", Jonathan Coulton @ Pop!Tech 2006 (more a series of songs than a lecture...).