I also highly recommend Leechblock [1] for Firefox. On my work machine, I have it set to give me an N second delay when visiting a long list of fun sites. As I watch the N seconds tick down, that's enough time for me to ask myself, "Do I really need to look at Facebook now?" Interestingly, N appears to be somewhat variable depending on circumstance. 10 seconds is enough normally, but when I'm not getting enough exercise or am dealing with work I don't want to do, N goes higher.
I also am currently loving Kanban Flow [2]. It's a Kanban board that has a built-in Pomodoro timer. (Briefly, the Pomodoro method involves working in 30-minute blocks with a mandatory 5-minute break. At the end it asks you if you've been focused; if you are, you get credit for the time.
[1] https://addons.mozilla.org/en-US/firefox/addon/leechblock/ [2] http://kanbanflow.com/
Note: When I was writing this comment and probably unintentionally DDOSing the Gmail servers together with who knows how many people by pressing F5, it came back to life.
Everyone burst into laughter. They had been talking about it for minutes.