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 amount of content in this game is surprising! Its free but not free to play meaning you have access to everything the game has to offer! Also there are lots of custom campaigns and other things in the games download section.