GemRB is a portable open-source implementation of the Infinity Engine
Game engine supporting many 3D/2D formats and graphic effects
Old-school 2D game engine
Orx: Portable Game Engine
Full featured MUD server with all the trimmings.
C++ lightweight cross-platform game engine
A game engine for console based text games
A Tile-based Game Engine written in C# mimicing retro consoles.
a UCI chess engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A cross platform Lua based game engine
Opensource Game Studio
Low-Level Game Engine for Retro Platforms
Portable game engine with main focus on point-and-click adventures
Web powered cross-platform 3D, WebXR game engine