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.
The best open-source and free game out there and also the best turn-based fantasy strategy game!