Berkeley courses on YouTube
As excited as I was when the MIT/OCW initiative grew to include most (now, all) MIT courses, I think this is much, much bigger. Materials are great, but seeing the videos there is what is actually putting the courses online.
Though education/knowledge is the biggest competitive advantage there is now, somehow as a civilization we've decided that the right thing to do is to share it. I've never really figured out how that came to be but the fact that it is just seems to make the world a better place, no? :)
Of the most desired courses though, I'm willing to bet the B-school courses will be the last (or at least offer the most resistance) to being put online. Extrapolating from the attitudes of many of my classmates, MBAs are going to try to hang on to that competitive advantage as long as possible.:)