UFO: Alien Invasion is “a squad-based tactical strategy game in the tradition of the old X-COM PC games.”
The game is open source and looks really nice (it is using a modified version of the Quake 2 engine). Development on UFO: AI is very active, and the developers do an awesome job of updating fans of the game on what’s happening development-wise.
UFO: Alien Invasion has a number of features that X-Com and its sequels never had (to my knowledge), including a cool multiplayer features that lets one player take the role of defending the Earth while another player gets to assume the role of the alien invasion force.
That’s pure unadulterated turn-based strategy goodness right there.