Battle for Wesnoth is a pretty nice open source, turn-based fantasy strategy game. Yeah, its a little old school, but I’m finding it incredibly addictive. It’ll run on just about any contemporary OS (Windows, OS X, Linux and a slew of others) and its released under the GPL (meaning, its free). Very much worth checking out if you have fond memories of Warlords, Miltary Madness or Age of Wonder.