Plug a Dell or LG into a Mac and the F1/F2 keys do nothing — Apple reserves real brightness control for its own displays. MonitorControl talks DDC/CI to the monitor over the existing video cable and remaps the standard Apple keys to actually move the panel’s hardware backlight, with the native macOS OSD overlay. It also slaves a non-Apple display to the built-in screen’s ambient-light dimming, so the laptop and the external can finally agree on what time of day it is. Free, open source, brew-installable.