3D game engine/framework in C, with Luajit and Python bindings now
High performance UI layout library in C
GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
Game engine supporting many 3D/2D formats and graphic effects
A game engine for console based text games
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
DOOM / Heretic / Hexen source port with enhanced graphics and UI
scriptable game engine with built-in support for 2D graphics and sound
Multi-platform game engine similar to old Megaman with a stage editor
A generic 2D engine for any kind of game
A truly extensible cross-platform game engine
Open source, moddable game engine for the "Polygon Love 2" game
Javascript adventure game engine
The Lambda-Engine is a cross platform game engine.