From Krylla Software:
Takeabreak helps you to take necessary breaks by blocking the computer at the intervals you decide. It prevents you from cheating and forces you to take the break that otherwise is so easy to leave out. You will likely find it very annoying sometimes but that's why Takeabreak is better than an alarm clock. Every break makes a difference for your body. It is a small program that is really easy to use. This version is the first release on CNET Download.com.