Having used Cold Turkey before, I can confirm that it's really good at doing what it set out to do (though I have a few annoyances with it, since there are a few things it doesn't allow me to do). What's impressive about it is that it is quite difficult to bypass it, and all of the naive approaches just won't work (eg. uninstalling it or switching to a different account).

I wish there were a Linux equivalent of it, but nowadays I don't need it as much as before anyway—I'm not sure if that's complacency, or if I just got better at focusing.