A physical timer, which operates on a single AA battery. Twisting the center knob resizes the coloured portion, which immediately starts shrinking as time progresses. A switch on the back enables an optional buzzer when the time runs out.
I first heard about this gadget from the Youtube channel How to ADHD. It's common for those with ADHD to fidget, and also have trouble with Time management. Thus, a physical object with an easily understandable interface we can fiddle with is much more appealing than a clock app on our phone or computer. In addition, using an app consumes crucial screen space I'd rather reserve for my work.
My only minor complaint was that the first time I used it, I was not aware it was counting down. It's completely silent, and the coloured area shrinks so slowly i had to wait around a minute to be sure it was on. I've been very happy with it ever since.
It frequently goes on sale on Amazon, and I bought mine for around $19.78USD including taxes. (Goodness US sales taxes feel deceptive 😠)