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.
While its quite challanging and overall pretty, i cant help but dislike staring at all the AI players making moves for most of the time. I know this is common for turnbased games but without fog of war and quite alot of units to move and each unit executing the same animation multiple times while attacking its just too much waiting =(.