Godot Engine - Feature-packed 2D and 3D open source game engine.
Castle Game Engine - An open-source 3D/2D game engine for ObjectPascal.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
Open 3D Engine - Open 3D Engine is an open-source 3D game development engine in real-time.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.