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.
This game is really cool. The gameplay is awesome and ranges from easy to very challenging depending on your difficulty settings. The graphics are great(they are 2d but the artwork is great). There is multiplayer if you like multiplayer games. It is COMPLETELY free and has a lot of add on content(also free). The game is fully playable and you can choose from stable versions or beta versions since the game gets updates still.