It’s fairly well known that long stretches at the computer without taking a break are unhealthy and often unproductive.
If you’d like something to remind you to take a break from work, and you use iGoogle, then try this work balance timer.
It’s a simple, free, customisable tool that counts down a specified number of minutes, then warns you when it is time to rest. It also informs you when it is time to start working again (d’oh!).
If you don’t use iGoogle, you should consider it as your home page. There are thousands of applications that you can install on your browser, which you can then conveniently access from any computer.