WorkPause

WorkPause is a home or office break reminder - a software program to remind you, when necessary, to stop working for a moment and have a rest break.

It has been proven that regular breaks help to avoid tiredness and eyestrain, and reduce the risk of developing RSI or other computer-related diseases.
How it works:
You can specify how much time you'd like to work between the breaks, when to have short or long breaks etc.
While you work, WorkPause sits in the system tray and shows you the time left until the next pause. It will monitor the intensity of your work and adjust the timing accordingly.
When it's time for a rest break, WorkPause will let you know; it can use sound with a small message, or black out the screen to make sure you really stop working.
Of course, if you don't like or are unable to stop working right now, you can always postpone or disable the alarms.