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.
Battle for Wesnoth is an excellent game. The gameplay is quite good, and the game possesses great art. And, what is rather nice, Wesnoth is open-source :)