3D game engine/framework in C, with Luajit and Python bindings now
Game engine supporting many 3D/2D formats and graphic effects
3D Engine c++
GemRB is a portable open-source implementation of the Infinity Engine
A modern, cross-platform Quake game engine based on FitzQuake.
Odamex is a free, cross-platform, modern client-server Doom engine
A cross-platform port of Hexen II game.
X3D is the open-standard format for 3D graphics scenes on the Web.
A free 3d game for PC that does not limit the player. Fight,Build,Gain
realistic driving simulation + functions for 2D/3D graphics
A real-time 3D Engine written in Ada
C++ lightweight cross-platform game engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Chess program
DOOM / Heretic / Hexen source port with enhanced graphics and UI
A fun realistic multiplayer free-and-open-source first-person-shooter!
Opensource Game Studio