Tasks 'n Timers
Free online timers
Pick a length and press Start. No sign-up, nothing to install, and each one ends with a chime.
How to pick a length
Short blocks make a task easy to start. Long blocks suit work that takes a while to warm up. If you aren't sure, start with 25 minutes: it's long enough to get something done and short enough that you'll actually begin.
Want breaks built in? The 52/17, 50/10 and Pomodoro timers move from work to break by themselves.