fheroes2 is a recreation of Heroes of Might and Magic II game engine
GemRB is a portable open-source implementation of the Infinity Engine
Game engine supporting many 3D/2D formats and graphic effects
A free 3D game engine to create games and realtimes scenes for Windows
Ze Text Adventure game Builder : Create as you play !
Orx: Portable Game Engine
ZDoom 2.8.1 fork focusing on modernisation, performance and stability
The world is no longer the same
Game/Scene Development System
Amateur 3D Game Engine
Full featured MUD server with all the trimmings.
Vivid3D is an open-source 3D engine with powerful 2D UI.
C++ lightweight cross-platform game engine
An isometric arcade-style game of skill, puzzles and exploration
A simple tool to allow Chessmaster 10/11 bot customizations
A fork of Chocolate Doom with small Quality of Life enhancements
A fork of dhewm3 with an emphasis on modernisation and vanilla Doom 3
Go 3D Game Engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.