Workrave is a program that assists in the recovery and prevention of
Repetitive Strain Injury (RSI). The program frequently alerts you to
take micro-pauses, rest breaks and restricts you to your daily limit.
These can be customized and it stops the counter when you stop.

The program can be run distributed on one or more PCs. All connected
PCs share the same timing information. When you switch computers, you
will still be asked to pause on time.

WWW: http://www.workrave.org/