Greasemonkey script, tested on firefox, in install click on the .user.js file then 'raw' if you have greasemonkey plugin or similar it should install.
Designed to get people who sit in front of the computer a reminder to get a drink of water and get away from the screen for a bit.
The basic one is done, it uses javascript alert, the full one will be a message box and disappear after a minute, or at least thats the dream.