I needed a simple timer for holding lightning talks, so I built one. You can see a demo of it at http://hackandtell.org/timer.
Andrew Gwozdziewycz originally made this for Hack and Tell.
Just dump all the files in a directory and load up index.html in your browser. If you append #10 to the url, you'll get a 10 minute timer. It defaults to 5.
Double click on the time resets it. Single click pauses/unpauses.