This handy dandy little piece of software nags you to take micro- and mini-break throughout the day. At real break times, it gives you stretches to do. At the end of the day, it tells you to get away from behind the computer and go do something else for a while. It also tracks your mouse movement, mouse clicks, and character strokes.
Wow! Is it GREAT for those of us who tend to get lost in what we are doing and lose track of time.
(one tiny warning: the default settings are too frequent and too many. Once I adjusted the settings to my own time-frames and limited the reminders for each one, I found it quite useful.)