Take Todobook’s method of restricting browsing time on a certain "unproductive" website, and apply it to any other website that you want. That is the concept behind SiteCop, a "productivity management" extension made for Chrome browsers.
First spotted by the folks at LifeHacker, SiteCop is an extension that let you block access to certain websites for a specific duration of time. Clicking on the SiteCop extension will open up a box that lets you type in websites and words which you would like to restrict access to.
Below the block list, you’ll find the timer settings that lets you dictate when the websites are rendered inaccessible. Fortunately, SiteCop does give some leniency as the "maximum minutes per day" box lets you set a specific amount of time whereby you’ll be able to access the restricted sites during their restricted periods.
Once the allotted time runs out though, the websites will be blocked until the restriction period ends, or if you decide to personally disable the extension.
In the event that the website you wish to block also has materials that are crucial to your work, SiteCop comes with a "Whitelist" tab. Copy and paste the URL of the page in question and SiteCop will make an exception for that specific page itself.
If blocking websites wholesale isn’t quite your style, SiteCop also comes with a "Pomodoro" tab that lets you cycle between work and rest periods. By default, SiteCop suggests 4 cycles that contains 25 minutes of work time and 5 minutes of rest time, but those numbers can be customized to your liking.
Once you’ve completed the set number of cycles, SiteCop will give you a longer break period that lasts up to 30 minutes before the cycle begins again.