Vivid3D is a C++ IDE/3D engine, with python scripting.
A free 3D game engine to create games and realtimes scenes for Windows
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
2D Game Engine
IRB is a game creation toolkit
Open Game Bootstrap
Gorgon Game Engine (GGE) is a C++ game engine that handles many tasks.
cross-platform 2d game engine
An extensible game engine that targets RPGs.
A small Game Engine project.