GemRB is a portable open-source implementation of the Infinity Engine
Real Time Strategy game engine supporting early Westwood classics
3D game engine/framework in C, with Luajit and Python bindings now
Game engine supporting many 3D/2D formats and graphic effects
Orx: Portable Game Engine
A modern, cross-platform Quake game engine based on FitzQuake.
Full featured MUD server with all the trimmings.
SDL based generic 2D game engine
Old-school 2D game engine
A game engine for console based text games
C++ lightweight cross-platform game engine
A Tile-based Game Engine written in C# mimicing retro consoles.
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
a UCI chess engine
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