I was working on someone else's system the other day and the monitor was just too bright for my taste so a friend reminded me of DarkAdapted X, a screen brightness control app which has a free and a pro version.
DarkAdapted is an application program. It controls the amount of red, green, and blue in your screen gamma so that you may, for example, preserve your dark adaptation while using your computer.
DarkAdapted is being used by astronomers, planetarium operators, graphics professionals, medical professionals, airline pilots, air traffic controllers, and others worldwide to provide flexible, dynamic control over their monitor's screen color response. Users may define an unlimited number of gamma presets, and invoke them via global hotkeys, by menu selection, or by typing the first few letters of a preset’s name.