But that App just adds redirects to your /etc/hosts file... Its pretty easy to replicate that behavior on Linux. I like OS X as much as the next guy, but that particular feature isn't really unique.
My problem is I have so little self control I just end up editing the hosts file. I need something with the same functionality that I don't know how it works so I can't circumvent it.
I just tried out SelfControl.app, and it's actually really effective! If you edit /etc/hosts while it's running, it replaces the entries immediately. And somehow I couldn't get any blocked websites to load even after quitting the app and then editing /etc/hosts. I'm not sure how it's doing it, but to be honest, it would be far less effective if I figured it out!