SunsetScreen is good if you live at an extreme latitude. Don't know if this has been addressed in recent updates, last time I used flux it wanted to start dimming my screen at 18.00 in the winter and close to midnight in the summer.

At the time, f.lux's devs seemed completely opposed to any kind of adjustable schedule for people who cannot follow the sun.

I use f.lux on Windows and it awesome.

Also, redshift is available for at least Linux, the BSDs, and an experimental version for Windows.

iOS and OS X has Night Shift, which is native implementation of the same idea.