Nice, does everything I need it for. Being able to define the redirect URL would be nice, but on the other hand, the default is amusing :)

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.