Lunar.fyi - Control monitor brightness, adapt using the ambient light sensor, adjust volume, switch inputs and turn off displays without fiddling with clunky buttons.
MonitorControl - Control your external monitor brightness&volume on your Mac
Duet Display - With Duet Display, you can transform your iPhone or iPad into a second display to expand the screen space of your PC or Mac system.
DisplayBuddy - Control the real brightness of your monitors directly from your Mac - no need to touch the buttons on your Monitor.
Screens - GNU Screen.
Air Display - Air Display allows extra screens to extend the display size of a Mac desktop. The company behind the product is Avatron Software, which was founded in 2008. Read more about Air Display.