There's a really nice, and free, extension for chrome called StayFocusd that I use.

https://chrome.google.com/extensions/detail/laankejkbhbdhmip...

You can mark sites you visit as "time wasters" (e.g. news.google.com, hulu.com, HN, etc.) as well as how much time/day you should be allowed to waste during work hours (customizable). It then logs the time you spend on those sites and after you've reached your daily limit, it will redirect you whenever you try to visit them.

Of course, you can work around it, but I find their redirect page wakes me out of an almost knee-jerk reaction to waste time throughout certain times of the day.