[cillian64]

I have a Dell U2415 which is great for this. You can create custom colour/brightness/contrast profiles and assign them to hotkeys. I have one nice bright one for dark terminals or when the sun's out, and a darker lower contrast one for when I have a bright white website or document to read and the office is a bit dimmer. It's two quick button presses to change between these presets.

I think I'd quite like something which adapted brightness and contrast to ambient light as well as the brightness and contrast of what's on screen. With mobile devices this can be a pain as you move around and in and out of shadows, but I feel it could work a lot better on a desktop display. The display could even have the light sensor on its back so it can work out what its backdrop looks like.