A series of four lectures on physics, specifically quantum electrodynamics, by Richard Feynman. Only Part 1 is available on Google Video and the rest are in streaming Real format (blech)...hopefully they too will make their way onto Google Video.
Update: Another lecture by Feynman, this one about Elementary Particles and the Laws of Physics.
Update: I got an email from the nice folks at Vega Science Trust asking me to change the wording of this entry with regard to encouraging people to put these copyrighted videos up on Google Video. Fair enough...what I really meant by that is I wish the videos were presented in a more useable manner than RealVideo format. If there's one thing that YouTube has shown us more than anything, it's that people find watching video in embedded Flash players really convenient.