StayFocusd is a Chrome extension that I've used for years whenever I feel like I'm falling into the bad habit of chasing rabbit holes on the internet. StayFocusd lets me specify the sites that I want to block from myself or limit my time spent viewing them. After specifying the sites and the amount of time I'll allow myself on them, a countdown timer appears whenever I view those sites. The timer resets every 24 hours.
Pomofocus is a task timer that is based on the Pomodoro method of getting things done. On Pomofocus I create a list of tasks then start the timer. Each task is allotted 25 minutes (you can adjust that). After 25 minutes there is a five minute break timer that automatically appears. After the break the next task timer appears.