Orx: Portable Game Engine
Gaming library
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
A generic 2D engine for any kind of game
A basic 3D environment with a first-person POV
TurboSphere is an easy to learn and powerful JS based game engine.
A very small multi-platform game engine for creating desktop games.
Open Source Virtual World and MMOG Development Platform
Cross-platform reimplementation of AMOS BASIC (runtime)
CM Game Development Kit