Open source clone of the Age of Empires II engine
3D Procedural Game Engine Using OpenGL
RTSish Game Engine
Retro game engine for creating games like it's 1997
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
An open source re-implementation of Diablo 2
KlayGE is a cross-platform open source game engine
An Adventure Game Engine
Community-driven game engine
Built to be ease of use, with no programming knowledge, Multi-platform
Free MMORPG Engine. Develop and host graphical browser-based games.