In 1964, the BBC filmed some lectures by Richard Feynman at Cornell called "The Character of Physical Law". They've been sort of hidden in their archives until recently, when Bill Gates purchased the rights to it. Microsoft Research has put said lectures online now as part of Project Tuva and they're a pretty good series of lectures.
Here's the site; it requires Silverlight (MacOS Intel, Windows).
Project Tuva: Feynman lectures online
Moderator: Alyrium Denryle
Re: Project Tuva: Feynman lectures online
Also works on Linux with Moonlight. Some of the .NET folks at Microsoft have been surprisingly nice to the open source Mono guys, helping them support Project Tuva and the Dynamic Language Runtime.