I found this awesome Add-On for Firefox (also works on Windows 7, Vista and iGoogle) called
StretchClock that helps alleviate the stress of working on the computer for long periods of time. Once you install the add-on, it will start a timer (default is 57 minutes, but you can change it for more or less time). When the timer goes off, a new window opens in Firefox and a 1 minute stretching video starts. Stop what you are doing, get up and do the stretch. You'll feel refreshed and less stress tension in your shoulders and neck. You can pause the timer - this is nice if you are using the computer to give a presentation or are in a meeting. You can also reset your timer if you've been out of the office doing other things.
Here are the instructions for installing StretchClock on Firefox:
