A "social" productivity extension for Chrome which allows you to blacklist websites. Once you have connected the extension with your Facebook account, it will post a status on your account if you visit a blacklisted site.
From there on you are at the mercy of your friends. If enough people like the status, Peer Pressure will close all the offending tabs. If someone comments with a URL, Peer Pressure will redirect you immediately to that site. This friend-induced inconvenience stops when you finally get control of yourself and go back to work.