I briefly used RescueTime's functionality for this. Eventually I actually settled on a separate browser profile for distracting sites. Basically, on my work Chrome, I only have productive sites, am not logged into any distracting services, or anything like that. I can use Chrome's user switching to open my "distracting" session, but the extra effort to do that helps me maintain focus.
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Related: http://blog.xkcd.com/2011/02/18/distraction-affliction-corre...
I briefly used RescueTime's functionality for this. Eventually I actually settled on a separate browser profile for distracting sites. Basically, on my work Chrome, I only have productive sites, am not logged into any distracting services, or anything like that. I can use Chrome's user switching to open my "distracting" session, but the extra effort to do that helps me maintain focus.