Break Timer keeps track of how much you are using the computer, and it reminds you to take regular breaks. It is designed to encourage healthy behaviour, with a mix of short breaks for your eyes and wrists, as well as longer breaks to keep your body moving and your mind on task.
Break Timer is easy to set up, so you can pick a break schedule that works for you and it will carry on working in the background.
Kinda pointless: there is no sound alert option, so while you're (looking) away, you need to time it yourself -- probably by looking again at another screen... (Btw, no idea what the previous reviewer talks about; it's only 3 MB.)