This is the best technical talk I've ever seen. Brett Victor, author of mind-expanding essays at worrydream.com, gives the clearest exposition of his philosophy in a talk at CUSEC while demonstrating some mind-blowing experiments in user interface. It is up there in quality with Steve Jobs' presentation of the first iPhone and the Mother of All Demos.
Bret Victor - Inventing on Principle from CUSEC on Vimeo.
More thoughts to come on why it is so good.