Everyone I know who has tried this program loves it. It is small and free. It adjusts your computer's display so that it's not blasting your eyes at night. When you try it, things will look a little weird at first. Then, you'll turn it off after a while and scream OH GOD NO and turn it back on, because you got used to having relaxed and happy eyes while computing in the evening.
There's a linux version now as well.