Monitorian - Monitorian is a Windows desktop tool to adjust the brightness of multiple monitors with ease.
ControlMyMonitor - View and modify the settings of your monitor from command line or GUI
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.
Dimmer - A very small and free utility for Windows to reduce brightness on LCD/TFT screens.
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