GemRB is a portable open-source implementation of the Infinity Engine
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Orx: Portable Game Engine
Multi Theft Auto is a game engine
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
KlayGE is a cross-platform open source game engine
scriptable game engine with built-in support for 2D graphics and sound
A roguelike game engine/editor
Open source, moddable game engine for the "Polygon Love 2" game
Tactical, turn-based game engine, that uses OpenGL and WinAPI