I use Crackbook Revival (Chrome) which is similar in purpose. However instead of giving you a countdown timer (because "but 5 more minutes, mooooom") it never actually stops you from browsing, but instead slows down select sites with every page load. First loading a page is instantaneous. 100th time takes seconds which is frustrating enough to cause me to go off and do something else, but without the same anxieties that Leechblock causes.