f.lux is a really handy tool that allows you to adjust the colour temperature on your display. If you stare at a screen for too long, then the glare from the display begins to hurt your eyes. To start with, turning down your monitor's brightness may help you, but brightness is not the really issue. The colour temperature is what causes the problems.
Most computer screens emit a bluish light, which looks great in daytime, but it becomes uncomfortable to look at when the sun dips below the horizon. Not only can your screen become uncomfortable to look at, it can also affect your sleep patterns*.
F.lux works by calculating the approximate sunset time for your location. At sunset, your computer screen will mimic nature, gradually warming up the colours. This, in turn, blends in better with the surrounding light.
If you spend a large amount of time looking at your computer screen, then f.lux is definitely something that you should check out.
*There has been research which suggests reading on a tablet for a couple of hours before you go to sleep can delay your sleep pattern by approximately an hour.