Grzegorz's Site Blocker

Very helpful for blocking sites I'm not interested in. I like how it now redirects to the Wikipedia page for Akrasia :)

This extension does what it says on the tin. Runs entirely in-browser (no HTTP requests), and is only about ~100 lines of code, find on GitHub under gregorias/grzegorzssiteblocker. The UI is a little rough, just beware that the box needs to be checked for the site to be blocked, and if any URL patterns are invalid then no sites will be blocked.