Webcasts have still a long way to go, in terms of quality (sound, etc), access and availability.
But some are really worth it:
from the GSD website (via archinect), take, for instance, Jeff Kipnis, Keller Easterling, and many others;
from the EDA/UCLA archive, there's this Kwinter lecture, and a Marcos Novak one from a series led by Casey Reas, and an impressive list of many others;
and from the Tate archive, don't miss (again, among many others) this great one from Manuel De Landa: Nature Space Society.
from the MIT Museum, check the "Soap Box" series, from which I strongly recommend this one on energy: "The Role of New Technologies in a Sustainable Energy Economy". The first 20 minutes are a must - clear, informative, mind-boggling.