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 free, Battle For Wesnoth is some of the best you are gonna get. Many custom campaigns, and enjoyable gameplay for just $0.00, yeah... It's hard not to find this game to be good.