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
Multi-platform game engine similar to old Megaman with a stage editor
Built to be ease of use, with no programming knowledge, Multi-platform
Open source, moddable game engine for the "Polygon Love 2" game
Gomoku Game: Playing with Computer