Battle for Wesnoth is a ton of fun
Battle for Wesnoth is an amazingly addictive 2-D turn-based strategy game with some role playing game elements thrown in for spice. It runs under Linux, Windows, and Mac OS X. Wesnoth can be played solo, using one of the several single-player adventures (campaigns) available, or over the Internet with other people.
The object of each round of Wesnoth is to complete a mission. In most cases, your goal is to destroy the opposing player's leader. When your leader is moved onto a base, you may call units to assist you which will form your army. Using your army, you must determine the best strategy to crush the opposing army and destroy its leader. Unfortunately, creating your army is not cheap; each type of unit will cost you a different amount of money, and the game gives you a limited supply of cash. If you move one of your units onto a village icon, you will begin drawing funds from that village for the rest of the match, unless the opposing player takes control of it from you. Each unit that you summon into the game will grow and advance as it defeats enemies, and the data for each unit is retained, so you can recall units in future missions in the same campaign or multiplayer match.