Not necessarily useful but really cool and I'd love a digital watch face that does this. https://sunclock.net/
Essentially it's just a 24 hour analog clock's hour hand over colors representing what the sun is doing. It really helps me understand when the sun is going to set so much more than a number.
I believe analog clocks are more useful for understanding the flow of time than digital clocks. Digital clocks have this thing where a few.minutes before the top of the hour can still feel like an hour (or at least somewhat) before the hour. Meanwhile with an analog clock it just looks really close. Heck, you might even misread it as being past the hour mark. So take all of that brilliant fuzziness and overlay it on a graphic representing what the sun is doing and it all just really comes together into a beautiful tool.