After reading Official Google Blog: Avoiding RSI I started looking for a good tool to avoid RSI (aka Repetitive strain injury, a disorder affecting bone and muscle from repetitive movements) and found Workrave. Now I can't stop using it:
It has 3 break timers:
1. Micro Breaks (asking your to stop and rest for a few seconds)
2. Rest Breaks (A few exercises follow by a few minutes away from the computer)
3. Daily Limit (Tells you when to stop working for the day)
All of the above are fully customizable and optional. If you spend a few hours or more on your computer you should try this great tool. It makes a big difference.