Game engine

Castle Game Engine (Pascal, LGPL-2.0, mature)
DelverEngine (Java, Custom, beta)
DesktopAdventures (C, LGPL-2.1, beta)
erampage (C, GPL-2.0, beta, inactive since 2015)
Flare (C++, Java, GPL-3.0, mature)
Free in the Dark (engine) (C, C++, GPL-2.0, beta)
FreeSO (C#, MPL-2.0, beta)
Grit Game Engine (C++, Python, MIT, beta)
ioquake3 (C, GPL-2.0, mature)
Jake2 (Java, GPL-2.0, beta, inactive since 2006)
JediOutcastLinux (C, C++, GPL-2.0, mature)
JFDuke3D (C, GPL-2.0, beta, inactive since 2005)
Librelancer (C, C#, MIT, beta)
Nuvie (C, C++, Lua, GPL-2.0, beta, inactive since 2018)
Odamex (C, C++, GPL-2.0, beta)
Open Cube (C++, zlib, beta, inactive since 2017)
Open Game Engine (C++, LGPL-2.1, beta, inactive since 2008)
openage (C++, Python, GPL-3.0, beta)
OpenJK (C, C++, GPL-2.0, beta)
OpenMW (C++, GPL-3.0, mature)
OpenNFS (C, C++, MIT, beta)
OpenSAGE (C#, LGPL-3.0, beta)
OpenTomb (C, C++, Lua, LGPL-3.0, beta)
PainTown (C++, Java, Python, 3-clause BSD, mature)
Pentagram (C++, GPL-2.0, beta, inactive since 2018)
Q-Gears (C, C++, GPL-2.0, beta, inactive since 2015)
RBDOOM-3-BFG (C, C++, GPL-3.0, mature)
ResidualVM (C++, GPL-2.0, beta)
Rigel Engine (C++, GPL-2.0, beta)
ScummVM (C++, GPL-2.0, mature)
sfall (C++, C, GPL-3.0, mature)
Spring (C++, C, Lua, GPL-2.0, mature)
TwinEngine (C, GPL-2.0, mature, inactive since 2015)
VASSAL Engine (Java, LGPL-2.1, mature)