I have been using Workrave at Day Job. This is a little application that “frequently alerts you to take micro-pauses, rest breaks and restricts you to your daily limit” of keyboard and mouse use.
At the rest breaks, a Laura Croft-like figure walks you through stretching exercizes. The breaks were a little annoying at first, but good discipline for those who are typing themselves into carpal tunnel syndrome.
It is available in English for Linux (.deb and RPM) and Microsoft Windows; sorry Mac users.