The Pomodoro Technique breaks work into focused 25-minute sprints separated by short breaks. Start this 25-minute timer, work until it rings, then take a 5-minute break before the next round.
Type a number of hours, minutes and seconds — or tap a preset — and press Start. The timer counts down and sounds an alarm when it reaches zero, even if you switch tabs (the tab title shows the time left). Pause and resume anytime, or Reset to start over. Space starts/pauses; Full screen gives a big, across-the-room display.
Counting down to a date or event instead? Make a countdown. Need to time how long something takes? Use the stopwatch.