i've tried eyedefender and 'take a break'. Liked them both. The former makes you exercise your eyes during your break via images on the computer screen. The latter has some handy features like deferring your break for another ten minutes with a click.
Workrave also has two break levels - called Micro Break (default interval between breaks is 4.5 minutes) and Rest Break (default interval is 30 minutes, iirc). The breaks can be deferred for a few minutes. Also, if one goes enough time without using mouse/keyboard, the time to next interval automatically resets. (workrave calls it natural break)
During the rest break it will show you exercises for eyes and arms. The exercises can also be accessed at any time by right clicking on the tray icon.
The best bit is the statistics records, which shows the number of mouse clicks, number of keystrokes and total mouse travel distance per day.
Workrave also has the ability to synchronize the timers on multiple machines that you use simultaneously, and are networked. I couldn't get this part to work without crashing workrave.
Link to website
