Godot Engine - Feature-packed 2D and 3D open source game engine.
Unity - The multiplatform game creation tools for everyone.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
ct.js - ct.js is a 2D game editor (desktop app) based on web technologies.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
BYOND - BYOND is the premier community for making and playing online multiplayer games.