EyeDefender helps by starting breaks at intervals of your choosing (you choose the length of the break as well). To rest your eyes you can choose either your default screensaver, an image, or the visual training mode. The training mode is a bit reminiscent of the relaxation portion of Flash Focus for the Nintendo DS.
If you stick to the schedule you set up, EyeDefender breaks will also help prevent repetetive strain injuries. Like anything designed to help your quest for improved health, you've got to stick with it to see benefits.
For web workers that have a hard time tearing themselves away from their screen, EyeDefender is a handy application with tremendous potential benefits. It's free for non-commercial private use and runs on Windows only.