What free software is there for observing the sky (sky map software)

Sky Chart / Cartes du Ciel has versions for different platforms.

KStars is a Linux application, but it can also run as a part of KDE on Windows. Otherwise, you can find it in the package repositories of your Linux distribution.

XEphem works on Unix-like systems, including Linux and Mac OS X.

Celestia, perhaps.

You could also try Google Sky.

See this question: Are there websites or programmes that permit a simulation of the night sky in the past and the future on an ordinary computer?

My favorite is Microsoft Worldwide Telescope (FREE), although it is probably on the high end of system requirements and battery drain. It is highly customizable though so there may be settings that can be tweaked to make it more power-efficient.