PyCAPGE - Python Classic Adventure Point and Click Game Engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Source code of the chapters of the book 3D Game Development with LWJGL
.NET - VISUAL BASIC NVIDIA +ATI PHYSX SLIMDX DIRECTX 9 -ENGINE/WRAPPER
Mahasona engine is an open source game engine.
XNA based game engine completely written in c#