Related: Try your phone on grayscale.
After adding the extension to Chrome, you create a blacklist of sites. Whenever you go to a site on the blacklist, you first encounter a gray screen that counts down before taking you to the desired site:
This give you time to reconsider, but since it's only a short delay you're unlikely to turn off the extension.
I've only used it for a couple hours this morning – so far it seems to be an elegant weapon in the "reclaim your time from the nefarious internet" kit.