F.lux – It makes the color of your computer's display adapt to the time of day (justgetflux.com)

I used this for a couple of months, and then removed it as I found it mostly annoying. I'd much rather have a little utility to manually adjust the tone of my display.

If you're looking for an open source version, check out RedshiftGUI at http://sourceforge.net/projects/redshiftgui/.

This is a great tool. One of the first things I install when configuring new OS. I recommended it to my friends and while some of them were skeptical at first they are using it since then.

Linux version of f.lux is called redshift. There is also app made for android but I never used it much.