Here's a
link to a small command line utility for Windows that uses SetDeviceGammaRamp() to adjust the brightness level of the display. Not all devices support this method but it seems to work well on Windows 7/8.
SetGamma v0.01
Purpose:
Sets the gamma level for the display.
Usage:
SetGamma <intensity level>
Where <intensity level> is number between 0 and 255.
Lower values are darker and higher values are brighter.
A value of 128 will return the display to the default brightness
Example:
SetGamma 224