Tombstone Engine - A direct successor to the C4 engine.
Godot Engine - Feature-packed 2D and 3D open source game engine.
Babylon.js - A complete JavaScript framework for building 3D games with HTML5, WebGL and Web Audio
PlayCanvas - PlayCanvas is an open-source game engine built on WebGL and WebVR.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
Armory3D - Armory 3D is a free and open source game development software developed using Haxe and Kha and made to run inside Blender.