StayFocused Chrome Extension

I came across this on Lifehacker, but basically, you install this extension in Chrome and it blocks distracting websites once you've reached a time limit.  You can customize which website become blocked and after how long.  Not very useful now that I'm a working adult (because we all know we'd go crazy at work if we couldn't distract ourselves with blogs, youtube etc), but I could definitely have used this when I was a student.