I made a productivity enforcing Chrome extension some time back. It allows one to prevent Chrome from opening websites based on a list that you can specify. If any site is opened when you activate it, this extension automatically closes them I’ve found it’s been pretty useful to have when trying to avoid procrastinating on the computer.
The previous version had some small issues making it non-distributable. I quickly whipped up a new version from scratch today and it’s available on the Chrome Web Store for free.