Godot is a powerful cross-platform game engine
The game engine for everyone
Urho3D Game engine
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
New Godot bindings for Swift
GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
2D Game Engine
Cross-platform 2D and 3D game engine
online 4X browser game engine
Egret is a brand new open mobile game and application engine
Cross platform 2D/3D game engine written in C++11 & OpenGL ES 2/3
2D physics engine for games
Cross-platform 2D game engine