From Parker Kuivila:
If you find yourself constantly ending up on distracting websites, you know what a time sink your computer can be. Unfortunately, you can't just stop using your computer because you need it to be able to get work done. There are tools out there that will block you from going online, but there are some sites that you made need to access. That's why I've made SelfRestraint; it blocks websites you pick for a set amount of time. That way you can continue to go online without the fear of getting distracted. Once it's running, there's no turning back until the time expires.