Desktop Dimmer is an open-source tool for setting the brightness of your monitor.
The program seems very basic. It added a system tray icon to our PC; clicking this displayed a brightness slider, and dragging the slider changed our monitor brightness.
There is a little more to it than that. Not only will the program work with all your monitors, but it'll automatically save and restore each display brightness setting without you having to do anything at all.
The other unexpected extra is Desktop Dimmer's massive drain on your system resources. It's an Electron application, essentially an entire browser, so we were expecting issues, but on our test PC it launched no less than four Chromium processes, grabbing a total of more than 240MB RAM. That's maybe 10x to 100x more than we'd expect for something this basic.