Free MARS!

The Battle for Wesnoth is a turn-based tactical strategy game with a high fantasy theme.

Freeciv is an open-source, turn-based strategy game based on Civilization II. Inspired by the history of human civilization, the game commences in prehistory and your mission is to lead your tribe from the stone age to the space age...

Widelands is a free, open source real-time strategy game with singleplayer campaigns and a multiplayer mode. The game was inspired by Settlers II™ (© Bluebyte) but has significantly more variety and depth to it. Still, it is easy to get started through playable tutorials.

FreeCol is a 4X video game, a clone of Sid Meier's Colonization. While remaining faithful to the original in terms of mechanics and gameplay, FreeCol sports a new set of redesigned graphics. Moreover, in addition to the classical Colonization rules, it features an additional ruleset that incorporates ideas that didn't make it to the final version of Meier's game, requests by fans and original concepts like new European players with new national bonuses.

C-evo is a freeware empire building game for Windows

Unknown Horizons is a 2D real-time strategy simulation with an emphasis on economy and city building. Expand your small settlement to a strong and wealthy colony, collect taxes and supply your inhabitants with valuable goods. Increase your power with a well balanced economy and with strategic trade and diplomacy.

TripleA is a free (under GPL) turn based strategy game based on Axis & Allies board game. A wide variety of map scenarios have been developed for TripleA (World War II, Punic Wars, Napoleonic wars, etc.) with a variety of rules, units and many options (such as "low luck" which reduces the number of dice rolled making game depend more on strategy).

Advanced Strategic Command is a free, turn based strategy game. It is designed in the tradition of the Battle Isle series from Bluebyte. ASC can be played both against the AI and against other human players, using hotseat or PlayByMail. It is also used to run the multiplayer universe Project Battle Planets.

Fulfill Your Quest For Global Domination

Ikariam is a browser based massively multiplayer online game of the strategy genre produced and maintained by Gameforge AG. The game is set in the era of classical Greece in an archipelago, with players being made tyrant of a small town, which they must expand and lead.

Ironfell is a time travel themed massive multi-player online real time strategy game. Travel back in time to plunder resources from the past, re-build your empire and conquer the stars.

Lord Ironfist is dead and the Kingdom is plunged into a vicious civil war by his feuding sons. At stake is the ultimate prize: control of the land and succession of the royal throne. Will you support the villainous usurper and lead the armies of evil or be loyal to the righteous prince and deliver the people from tyranny.