Twinkle Tray lets you easily manage the brightness levels of multiple monitors. Even though Windows 10 is capable of adjusting the backlight on most monitors, it typically doesn't support external monitors.
ControlMyMonitor allows you view and modify the settings of your monitor (Also known as 'VCP Features'), like brightness, contrast, sharpness, red/green/blue color balance, OSD Language, Input Port (VGA , DVI , HDMI ) and more...This app doesn't have any features at the moment.
iBrightness Tray is a little bit of a swiss army knife display tool as adjusting the brightness of your screen is just one of three main functions it has. The other two are turning on your screensaver at the click of a button and also the ability to turn off the display...Discontinued
The program is no longer updated. Last version, 1.0, released in 2013, can be still downloaded from the official website.