Workrave. Cross-platform, simple, wonderful.
I found WorkRave increased my stress 10 times and I just ignored all it’s breaks in the end.
Eyes Relax is far better and less annoying than Workrave. You can set a picture, slideshows, custom noises etc.
Eye defender is another good one but has less options.
The best system is to use the very simple and effective Pomodoro time management technique as it teaches the true productivity benefits for working in 25 min block with 5 min breaks (trust me it will change your work life)
Once you’re using that method you can use this windows gadget to make sure you don’t spend to long at your pc or too long far away 😉 There is one for mac i’m sure.
Well, its not an RSI watcher, but when Im seeing a movie or some tv show I strengthen my wrists by using a powerball.
It can be dangerous if you have a curious dog with a tongue =)
A colleague of mine has a roster in his Outlook Calender that alerts him every half hour or so, he just moves it along days so he can reuse it. Rather clean and simple, though not as elegant as an app designed for workbreaks