An OpenGL RTS game engine written in C
GemRB is a portable open-source implementation of the Infinity Engine
A cell-based puzzle game featuring classic and completely new puzzles
PyCAPGE - Python Classic Adventure Point and Click Game Engine
DOOM / Heretic / Hexen source port with enhanced graphics and UI
KlayGE is a cross-platform open source game engine
Browser game engine
The Lambda-Engine is a cross platform game engine.