I've been using an app called Concentrate (http://www.getconcentrating.com/) for probably 5 years that does the same thing, but it also lets you define apps and services to block (so for example, if I want to get really focussed, I block my Mail app so that I can't check it for the next hour).
I had the pleasure of working with Made By Rocket (makers of Concentrate) when I was a senior in high school (I was 17). Living alone in downtown Boise Idaho, working 40 hour weeks for two different startups, and finishing high school, man that was a rough year.
Concentrate is great, but I'm going for something a little different with Focus over the long-term. Hope to have more features out soon that make this clear :)
