Most of us lead sedentary lifestyles these days -- most of our time is spent in front of computers. This slowly is causing a lot of problems people from previous generations haven't experienced: back aches, knee problems, wrist pains, myopia, among others. And just going to a gym or putting in one hour of physical activity a day isn't enough. It doesn't help balance the inactivity over the entire day.
I've tried both the software but have been using Workrave for quite a while now and am quite happy with it. To briefly introduce them: both software prompt the user to take a break at regular intervals. They have timers that trigger at configured intervals asking the user to take a break. Workrave also has some stretching exercises suggested that can be performed in the longer breaks. The shorter (and more frequent) breaks can be used to take the eyes off the monitor and to relax them. Read the article for more details.
I've reviewed Workrave version 0.9.1 in the article, though the current version as of now is 0.9.3, which has a few differences from those mentioned in the article. The prime difference is the addition of a 'Natural Rest Break' that gets triggered when the screen-saver gets activated, which is nice since if the user walks away from the computer for a prolonged period of time, the rest break in effect has been taken, and the next one is scheduled after the configured duration once the screen-saver is unlocked.
Both software are available in the Fedora repository: Workrave is based on the GTK toolkit (and integrates nicely with the GNOME desktop), whereas RSIBreak is based on the Qt toolkit (and integrates nicely with the KDE desktop). Give these software a try for a cheap but effective way of staying healthy!