Your computer screen usually stays at the same brightness and colour temperature all day - it doesn't take into account different times of the day and different lighting situations. This can end up being quite hurtful to those who have sensitive eyes.
f.lux is an app that makes your computer screen easier on the eyes in different lighting conditions and times of the day. For example you may be on the computer in the dark at night, it's better to have low brightness and a warmer coloured screen - f.lux takes care of that for you.
f.lux takes care of the changes for you depending on your settings. So you can add timings, lighting conditions and even your location. Then you just leave it and it'll do it all automatically. In the daytime it'll usually give you a cold look with full brightness, and at night it'll be low brightness with a warmer colour.
This brilliant program is free for Mac OS X, Windows and Linux. You can also get it on iPhone or iPad but you will need a jailbroken device.