GemRB is a portable open-source implementation of the Infinity Engine
Javascript library for making tile based games
2D Game 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
Java game engine for shoot 'em ups, using Java2D.
Cross-platform 2D game engine
Gomoku Game: Playing with Computer