What videos should everybody watch? Stanford University now has a Youtube channel, with free access to HD video of full courses on everything from dynamical systems to quantum entanglement.  More conferences and workshops are videotaping their talks.  What are videos online that you think everyone should know about?

I'll seed this with a few answers to presentations that are mostly expository, but what I'm hoping might happen is that this community wiki could turn into a resource to share excellent presentations of new research, as well as a place to learn (or reinforce) background in an unfamiliar area.
Timothy Gowers has a set of videos on Computational Complexity and Quantum Computation online.