Construct Classic - Construct Classic is a free, DirectX 9 game creator for Windows, designed for 2D games.
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.
RPGBoss - Cross-platform 2D role playing game maker.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
Adventure Game Studio - Adventure Game Studio provides the tools to make your own adventure, for free!