Morphine is a free Chrome extension which enables limiting access to distracting websites.
Setup starts by manually entering your most time-wasting domains - facebook.com, youtube.com or whatever they might be - and these are immediately blocked, with no emergency exceptions or ways around the protection.
Carry on working, though, and after every ten minutes (by default) Morphine will give you one minute to access the blocked sites.
This is convenient if you really need to check a site, because you're only ever 10 minutes away from being able to do so.
But it also encourages you to work for longer and build up your allowance, maybe to earn a six minute break every hour.
Morphine is a nicely balanced, well-designed site blocker, strict enough to block your most time-wasting sites, but with the flexibility to give you a few occasional breaks.