The Battle for Wesnoth is a Free, turn-based tactical strategy game with a high fantasy theme, featuring both single-player, and online/hotseat multiplayer combat. Fight a desperate battle to reclaim the throne of Wesnoth, or take hand in any number of other adventures. Easily modifiable. You can even download other users' add-ons from the main menu.
For people who like turn-based strategy games this is a must-play game.
The interface is superb providing all the needed info in a glance, displaying movement range, the possible out come of attacks, stats of the warriors,...
Due to its great replayability, this is definitely one of my all time favorites.