You. You who are reading this blog post.
You have already spent an ungodly number of hours sitting down, hunched forward, staring unblinking into this screen.
Do we really need your computer to remind you that you need a break every now and then? Sadly, yes.
MacBreakz (site | YouTube) is the more sophisticated of the two. I used this software extensively when I was spending 4-6 hours a day writing my thesis. It offers a good variety of stretches and ergonomic tips. It also monitors the intensity of your activity; if you're mousing around a lot or furiously typing to beat a deadline, it will pop up a reminder to take a micro-break of a few seconds. It nags because it loves. It's more expensive ($25) than most shareware, but worth it if you spend a lot of time on the 'puter.
Joe Ergo (App Store) is lighter weight, but also much cheaper ($2). Set a time for when you want to receive a reminder to take a break and then minimize the window. Joe Ergo offers fewer stretches, but that's OK -- it's the break that counts. I set the interval at 25 minutes (yes, like the Pomodoro). If I'm only on the MikeBook for an hour or so on a weekday night, then I'll use Joe Ergo.
I guarantee you will be surprised at how swiftly the interval you set passes and how irritating you will find these reminders. Starting the software and setting the interval is your calm, wise self looking out for the impulsive, in-the-moment, future self who wants to keep clicking those YouTube links until well past your bedtime.
In addition to giving my body a break, I also want to give my eyes a break. One of my eye doctors said that, when you stare into the distance, the muscles that focus your eyes relax. But when you focus your eyes onto a bright screen only a foot or so away from your face, the muscles tense up and "go all out like a Masseratti." Both programs above remind you to do eye exercises as well.
I also put wetting drops in my eyes when I take my break. My cataract doctor recommended keeping a wetting agent of some kind nearby and to use it often. He said he was horrified to see his wife working on her laptop and not blinking for minutes at a time. As he explained it, when the eye is moist, its surface is smooth as a marble. But when you stop blinking, the eye's surface begins drying out here and there and the surface gets rougher. The result is more refraction of light, glare, and squinting.
I recommend GenTeal liquid gel drops; I keep a bottle by my monitor at work and at home. It's tough to remember to do it regularly, but your eyes will thank you.