I have always loved commencement addresses, and John Doerr's for the 2007 class at Rice was the best serious commencement I have ever heard. For humorous ones, unfortunately this one does not have a video, but there is audio of David Brook's address (whose message is serious, however) on the right side of this page.

Edit: And probably the funniest commencement ever, Conan O'Brien's to his alma mater.