David Letterman wasn't always the brilliant talk show host he is today. He started off as a local weatherman before going to Hollywood to hone his acting chops.
Here he is doing light comedy with Michael Jackson. (Yes, you read that right.)
And this is him as an obnoxious self-help guru on Mork and Mindy.
That was a great find for me personally, because for years I've remembered this phrasing of "Sit down and shut up" and didn't know where it came from. Now I do. Thanks, YouTube! *grin*