If you are looking for a good program that is free, open source and useful for ensuring that you get off your computer periodically, look at workrave at http://www.workrave.org. You can set it up to tell you when to take breaks, perform some exercises during breaks and also maintain an ergonomically healthy computing habit. Install it on your Ubuntu system using:
sudo apt-get install workrave <enter>
or from the Ubuntu Software Center. Workrave is also available for other operating systems.