Well this is the latest trick I'm pulling on myself, and I can say it does help, but due to its nature YMMW: https://chrome.google.com/webstore/detail/go-fucking-work/hi...
I find https://chrome.google.com/webstore/detail/momentum/laookkfkn... and https://freedom.to/ to be better. The red is harsh and makes you feel ashamed, which makes you want to escape into the world of reading about the Great Boston Molasses Flood.
Shame is one of the most powerful tools to get us to change behavior, which is why it's so naturally ingrained in our brains. But again some people find it too harsh to the point they work less and/or may even have so little self-discipline that they'd disable the add-on; so my "YMMV" message stands :).