Godot Engine - Feature-packed 2D and 3D open source game engine.
Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
Lumix Engine - Open source 3D game designer and engine written in C++.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
MOS - Lightweight game engine.