AntiRSI is a simple program that reminds you to take breaks while working at your computer. There are two levels of breaks: "micro pauses," 15 second intervals that occur every four minutes, and "work breaks," eight minute periods that occur every 50 minutes. You can of course adjust how frequently these breaks happen.
It's a nice piece of nagging: if you start typing or mousing before the break is over, it starts again. You can also postpone the larger breaks if you're in the middle of something that needs to get done right. Since it's nagging my right now, I'll take a screenshot:
The program is small to download ( less than 100K), takes almost no memory and does the job very well. I've noticed a difference since I installed this. Just looking away from the screen for 15 seconds frequently does wonders for shoulder cramps and eyestrain.
AntiRSI from Onne Gorter in the Netherlands - Freeware