Game engine

Castle Game Engine (Pascal, LGPL-2.0, mature)
DelverEngine (Java, Custom, beta)
DesktopAdventures (C, LGPL-2.1, beta)
Free in the Dark (engine) (C, C++, GPL-2.0, beta)
Open Game Engine (C++, LGPL-2.1, beta, inactive since 2008)
OpenMW (C++, GPL-3.0, mature)
PainTown (C++, Java, Python, 3-clause BSD, mature)
ScummVM (C++, GPL-2.0, mature)
TwinEngine (C, GPL-2.0, mature, inactive since 2015)
VASSAL Engine (Java, LGPL-2.1, mature)