Nudge от Jacob Lurye

Nudge helps you stay focused online.

Set a target webpage — the one you actually want to focus on — then block the sites that are distracting you away from it. Next time you wander to a site that you've blocked, Nudge will send you right back to your target page.

Manage your target and blocked sites or temporarily deactivate Nudge at any time.

Nudge is open source. Feedback or contributions are most welcome: https://github.com/jacoblurye/nudge