Links to Online Videos of Good-Quality Lectures by Mathematicians
The audio isn't great but Serre's "Writing Mathematics Badly" is classic. It's a discussion of what is and what is not good mathematics notation, style, etc. It also has quite a bit of humor mixed in:
Part 1
Part 2
Part 3
Let me contribute by posting the following links:
Part 1
Part 2
Part 3
Part 4
Part 5
Part 6
It is a talk by Paul Cohen about his work on the Continuum Hypothesis and his interactions with Kurt Gödel. This talk took place at the Gödel Centennial conference in Vienna in 2006.