I did hint about it in another thread, but this is a rather simple, yet curious and surprisingly nice tool.
http://www.stereopsis.com/flux/

All it does is change the colouring of the screen depending on what time of the day it is.  Generally, making the screen a bit more warmer or more orange at night, and more bluish during the day.
I think the idea is that generally, at night, most people have warm coloured lights, so making the screen of a similar colour makes it easier on the eyes.

It might not sound like much, but try using it for a while, and then try disabling it at night - the difference can be rather astounding.