3D game engine/framework in C, with Luajit and Python bindings now
A modern, cross-platform Quake game engine based on FitzQuake.
GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
A Tile-based Game Engine written in C# mimicing retro consoles.
C++ lightweight cross-platform game engine
SDL based generic 2D game engine
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
scriptable game engine with built-in support for 2D graphics and sound
Multi-platform game engine similar to old Megaman with a stage editor