Monitorian - Monitorian is a Windows desktop tool to adjust the brightness of multiple monitors with ease.
Monitor Bright - Monitor Bright reduces with a simple trick the monitor brightness.
Twinkle Tray - Easily manage the brightness of your monitors in Windows from the system tray.
PangoBright - Screen dimming software that allows you to control the brightness of your laptop screens as well as external monitors.
f.lux - f. lux is a piece of software for Windows, Mac, Linux, iOS, and Android that provides a blue light filter over your screen. Research suggests limiting our exposure to blue light in the hours before bedtime can help us fall asleep faster.
LG OnScreen Control - Allows to set the brightness (and other settings) of the monitor through DDC/CI. Meaning: keyboard and/mouse instead of having to fiddle with the OSD menu manually. http://www.lg.com/us/support/monitors