Adventure Game Studio - Adventure Game Studio provides the tools to make your own adventure, for free!
Godot Engine - Feature-packed 2D and 3D open source game engine.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
Wick Editor - The Internet's free and open-source creation toolkit.