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.
I've been playing this game for a long time now, since it was first added to the Ubuntu repos and when it was still crude but incredibly entertaining. It still is, and the amount of user content made is astonishing. There are also Android and iOS versions. Excellent job for the people behind one of the best free open source strategy games out there.