ezEngine is an open source 3D game engine with a complete editor written in C++ using the Qt framework. It is hosted on GitHub and available under the MIT source license.
Details of the ezEngine:
ezEngine is an open source C++ game engine in active development. It is currently mainly developed on Windows, and higher level functionality such as rendering and the tools are only available there, but the core libraries are also available for other platforms such as Mac and Linux.
ez is built in a modular way, enabling users to either use all available functionality, or to pick and choose individual features and build the rest yourself. Larger features are implemented through engine and editor plugins and can therefore be easily removed or replaced. For instance sound (Fmod), physics (PhysX) and particle effects are all provided through runtime plugins.
The ezEditor is a full blown editor used for editing scenes and importing and authoring assets.