Being someone who gets easily sucked into the depths of the internet and loses track of time (damn you hyperlinks!), I have to finally admit I'm in need of some gentle reminder to take regular breaks from staring at the screen. I bet you haven't done that in the last 60 minutes either (if you happen to be an internet addict that is). Well, there is help in the form of a little gem of a software called Workrave. Workrave let's you set reminders to take frequent micro-breaks, breaks, and even set a daily limit of computer use.

Micro-breaks are meant to rest your eyes and relax for a few seconds, maybe every 10-15 minutes, you set the frequency. You can schedule full breaks at a longer interval and useful exercises pop-up on the screen to stretch your neck, relieve eye tension and more (there are 10 exercises in total). You can set the amount of exercises per break and they are selected randomly. Finally, you can even select a total time of daily computer use, after which you'll get increasingly urgent reminders. While taking your breaks you can have Workrave block any input (so you don't get tempted) and even have your wallpaper hide all open windows.

The software notices when you are active on the computer, so breaks are only counting down if you abstain from touching that damn thing. Also, if you leave your computer running while stepping off, Workrave will stop the countdown to your next break time, and resumes it once you return to your computer. That way the breaks are scheduled in consideration of your actual time staring at the screen.

All breaks can be freely set up to your own needs and willingness to be nagged. At the very least it will induce you to take some very healthy breaks, which will actually increase your productivity in the course of the day (or at least make your time-wasting surfing habits a bit healthier).
Workrave is free and open-source, runs on Windows and Linux (maybe on Mac as well).