FocusOwl от Erlend Eelmets

Improve your productivity. Set specific modifiable work and rest time goals. The extension keeps track of them for you and plays a sound to notify of you about meeting your goal.

FocusOwl is based around Pomodoro Technique.

The Pomodoro Technique is a time management method that involves breaking your work into intervals, typically 25 minutes in length, called "Pomodoros." After each Pomodoro, you take a short break of around 5 minutes. After completing four Pomodoros, you take a longer break, typically 15-30 minutes.