Shades

"Perhaps you need to use your machine in a darkroom, or like to work late at night without disturbing your partner who sleeps beside you?
Perhaps you need to dim the screen on a desktop machine, and the display you are using offers no such facility?
If this is the case for you, then try Shades.

Note: Shades may interfere with colour calibration software, and should not be used if colour accuracy is critical. "

quoted from the homepage