John Cornwell made it onto Letterman by making a refrigerator that will get you a beer. Two links to the Letterman appearance on YouTube (always make a backup, right?):
If you don’t have 2 1/2 minutes to watch the video you can read a little more at http://en.wikipedia.org/wiki/Beer_Launching_Fridge.
Conwell’s home page, http://www.duke.edu/~jwc13/beerlauncher.html, was last updated just before he went on the show. I guess he either graduated or no longer has time for a website because he’s so busy making Beer Launchers.