A Time Timer.
They're not cheap, especially for a timer that's bare bones (~20 USD).
But it has changed my work life.
- People who want to interrupt me while I'm working can now see how long until I have my next break. So I am interrupted less.
- Now I self-regulate a bit better, so I'm able to work longer without destroying myself in the process. I take breaks that help me with repetitive strain injuries and with feeling like I'm a human being and not just a machine.
- Now I remember to actually start timers when I start working. I know this is a bit silly, but I was having trouble creating a habit of stopping for breaks. I tried to solve this by setting timers on my phone, but I constantly forgot doing it. Now I'm reminded to start a timer by something that I see on my desk.