StayFocused
“You sit down at the computer, and you swear you’ll be productive. Next thing you know, it’s twelve hours later. You’ve checked your email, updated your Facebook status, browsed the trending topics on Twitter, read your RSS feeds, [read Reddit, my drug of choice,] looked up your favorite band on Wikipedia, vanity googled yourself, cyber-stalked your ex, looked at all your high-school crushes’ Facebook photos, watered your plants on Farmville, and lost a week’s pay playing online poker.
What you haven't done is WORK. StayFocusd is a productivity extension for Google Chrome that helps you stay focused on work by restricting the amount of time you can spend on time-wasting websites. Once your allotted time has been used up, the sites you have blocked will be inaccessible for the rest of the day."
Of course, you have to be running Chrome.