3D game engine/framework in C, with Luajit and Python bindings now
GemRB is a portable open-source implementation of the Infinity Engine
SDL based generic 2D game engine
a UCI chess engine
A cross platform Lua based game engine
scriptable game engine with built-in support for 2D graphics and sound
Flexible and easy-to-use 2d game engine
Multi-platform game engine similar to old Megaman with a stage editor
A generic 2D engine for any kind of game
Open source, moddable game engine for the "Polygon Love 2" game
Gomoku Game: Playing with Computer