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)
Heroes of Wesnoth (C++, beta)
LGeneral (C, GPL-2.0, mature)
LordsAWar! (C++, GPL-3.0, stable)
Mechanized Assault & eXploration Reloaded (C++, C, GPL-2.0)
MegaGlest (C++, GPL-2.0, mature)
Megamek (Java, GPL-2.0, mature)
NetPanzer (C++, GPL-2.0, beta)
Open General ()
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)
Tanks of Freedom (GDScript, MIT)
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)
Zero-K (Lua, GPL-2.0)