UFO: Alien Invasion is a computer game in which the player fights aliens that are trying to take control of the Earth. (ZPL)

The Battle for Wesnoth is a turn-based strategy game with a fantasy theme. (ZPL)

Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization. (GPL)

FreeCol is a turn-based strategy game based on the old game Colonization, and similar to Civilization. (GPL)

FreeOrion is a great, open-source, space-set, turn based strategy game. (GPL)

Glest is a free 3D real time strategy game, available for several operating systems and that can be modified using XML and a set of tools. (GPL)

Globulation 2 is an innovative Real-Time Strategy (RTS) game which reduces micro-management by automatically assigning tasks to units. (GPL)

Hedgewars is a turn based strategy game but the real buzz is from watching the devastation caused by those pesky hedgehogs with those fantastic weapons! (GPL)

Scorched 3D is a game based loosely (or actually quite heavily now) on the classic DOS game Scorched Earth "The Mother Of All Games". (GPL)

Spring is an open source project to develop a new realtime strategy game. (GPL)

Warzone 2100 is a real-time strategy game, originally developed by Pumpkin Studios (Archived website) and published by Eidos-Interactive. (GPL)

Widelands is a slow-paced real-time strategy free computer game under the GPL. (GPL)