It’s maybe a niche thing, but I use Unifed Remote a lot. It runs easily as little server on my various computers, and let’s me use my phone and smart watch as a control for them. Mostly used to control media (watching movies, adjusting white noise volume for sleep) but also work (controlling presentation, etc). It’s has various remotes from keyboard and mouse to just a simple media start/stop/volume/etc up to a remote screen display.

I sure there’s other apps out there that are similar, but it’s always just worked for me and seems simple enough for what I do. I paid for the full version (not sure if I even use any paid features, the basic had most of what I use) but if there’s a good Foss alternative out there I’d be happy to try it!