If your computer is your second home, like mine is, you might, from time to time, have a power-nap at the computer while you're waiting for that defrag job to finish. I recently found an online alarm clock that can wake you up in either 15 minutes, 30 minutes, 45 minutes or 60 minutes.

Amusingly called "The Naked Alarm Clock", it is a simple clock which shows the time on your computer and allows you to set an alarm to wake you up again. You can choose from four different alarm types and is easy to use.