PC monitor brightness controller (control the actual monitor's backlight!) supports multi-monitor.

Brightify is an OS-independent application that adjusts monitor brightness using the DDC/CI protocol and custom protocols for USB monitors, featuring a brightness sensor for automatic adjustments based on ambient light, and can be controlled via a taskbar icon.

Control your monitor brightness on Windows like you do on macOS

External monitor brightness Manager

Lumi is a Node.js module that allows you to adjust the brightness of your internal and external monitors.

This GitHub repository houses the Arduino Light Sensing Monitor Brightness Controller project. The project is designed to dynamically adjust the brightness of a monitor based on the ambient luminosity in the room, providing an intelligent and energy-efficient solution for managing display brightness.