3D Procedural Game Engine Using OpenGL
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Free, open source game engine online
Entitas is a super fast Entity Component System (ECS) Framework
KlayGE is a cross-platform open source game engine
Text Based Game Engine and Parser
Tactical, turn-based game engine, that uses OpenGL and WinAPI
Open source, moddable game engine for the "Polygon Love 2" game