Lunar.fyi - Control monitor brightness, adapt using the ambient light sensor, adjust volume, switch inputs and turn off displays without fiddling with clunky buttons.
Twinkle Tray - Easily manage the brightness of your monitors in Windows from the system tray.
DisplayBuddy - Control the real brightness of your monitors directly from your Mac - no need to touch the buttons on your Monitor.
Dimmer - A very small and free utility for Windows to reduce brightness on LCD/TFT screens.
BetterDummy - BetterDummy creates a virtual display which you can utilize as a mirror main for your display on the Mac.
PangoBright - Screen dimming software that allows you to control the brightness of your laptop screens as well as external monitors.