FreeCiv!

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.

Fulfill Your Quest For Global Domination

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.

Here comes the Golden Age of Civilizations, a semi turn-based MMO strategy game, inspired by but not following the classic games of the genre, such as Age of Empires, Civilization and Rise of Nations; mixed up with MMO capability and flavored with the best parts of other good online games.

C-evo is a freeware empire building game for Windows

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.

Crown of the Gods™ is a Free-to-Win MMO real time strategy game made by Gaming Addict Studios that is not, and never will be, pay to win. Crown of the Gods™ just finished its 3rd Closed Beta and will be going into Open Beta on January 23rd, 2016.

A board game for the web!

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.

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).