Workrave
Workrave is a free software application intended to prevent computer users from developing or aggravating occupational diseases such as carpal tunnel syndrome, repetitive strain injuries, or myopia.
The software periodically locks the screen while an animated character, “Miss Workrave”, walks the user through various stretching exercises, and urges them to take a coffee break.
The program is cross-platform, and dependent on the GTK+ graphical widget toolkit, and also on other GNOME libraries on Linux. It is also available for Microsoft Windows.
Latest stable release
As of December 2014: The official Workrave website implies that the latest stable Workrave release is 1.10, but this is incorrect. In fact, the latest stable release for Linux is 1.10.5, and the latest stable release for Windows is 1.10.1. The Windows uninstaller was broken in 1.10 and fixed in 1.10.1.
Similar software
Similar RSI-prevention utilities include:
pokoy, works on Linux
RSIBreak, works on Linux
Xwrits, works on Linux