Tuckit.

A timer almost always lives somewhere else: another tab, another window, a phone face down on the desk. Every glance at it is a small interruption, and the tab you actually need gets buried behind it.

Tuckit tucks the timer into the edge of the page you are already on. Pull it out when you want it, push it back when you don’t. It runs a plain countdown or a full Pomodoro, and like the rest of the Notoo family it is free, needs no account and asks nothing of you before it starts.