The Battle for Wesnoth is an RPG medival-style game for all operating systems. Not only is it free, but it's also open source. I really don't know what to say other than it addicted me for hours...and it's free.
If you're running Linux, you can get an old version of the game (at least on Ubuntu) by typing
sudo apt-get install wesnoth
into a Terminal. However, don't play The Southern Guard campaign if you do so. If you would like to, manually get the Debian packages that contain the newer version.