I get up very early in the morning, typically around 4am. The house is pitch black, so when I open my computer it vaporizes the top layer of my cornea. Even at its lowest setting the laptop screen is uncomfortably bright, but I learned something new yesterday which will save my eyeballs. On MacOS X, pressing control+alt+command+8 reverses the video. This is done system wide, at a low level in the video subsystem. No support in applications is required.
(simulated screenshot)
The overall effect is much more comfortable in dark environments, albeit somewhat jarring with images. Pressing the key combination again toggles it back. The video also reverts to normal when the laptop is closed, presumably as a failsafe in case it is engaged by accident.