1)   WorkPause 1.4.1
The WorkPause utility can help you prevent the tiredness, back pain, eye strain, repetitive strain injury (RSI) or other illnesses provoked by long uninterrupted work with computers, by reminding you to take breaks at regular intervals.

 2)   ScreenRest 2.0.0
ScreenRest is a straightforward and easy-to-use application which reminds you to take regular rests from your computer. Using ScreenRest gives you complete control over how often, and how long, you wish to rest.