Strategy games

0AD C++, GPL-2.0, alpha
Advanced Strategic Command C++, GPL-2.0, mature
Battle for Wesnoth C++, GPL-2.0, mature
Birth of the Empires C++, Private use allowed (original artwork under special license)
C-evo Delphi Pascal, None found, mature, inactive since 2013
Crimson Fields
Free Orion C++, Python, GPL-2.0, beta
FreeCol Java 8, GPL-2.0, mature
FreeRails Java, alpha
Freeciv C, GPL-2.0, mature
Freeciv WebGL Javascript, Java, AGPL-3.0, mature
Glest C++, GPL-2.0, mature, inactive since 2009
Globulation2 C++, GPL-3.0, mature
LordsAWar! C++, GPL-3.0, stable
MegaGlest C++, GPL-2.0, mature
Megamek Java, GPL-2.0, mature
NetPanzer C++, GPL-2.0, beta
Open Imperium Galactica Java, GPL-3.0, beta
OpenRA C#, GPl-3.0, mature
OpenXcom C++, GPL-3.0, mature
Pax Britannica Lua, C, MIT, mature
Spring RTS engine C++, C, Lua, GPL-2.0, mature
Star Control II: The Ur-Quan Masters C, GPL-2.0, mature, inactive since 2011
TripleA Java, GPL-2.0, mature
UFO2000 C++, GPL-2.0
Ufo: Alien Invasion C, C++, GPL-2.0, mature
Warzone 2100 C++
Widelands C++, Lua, GPL-2.0, mature
Xconq GPL, mature