FlatRedBall - Cross-platofrm 2D game engine using C#, focused on developer productivity, transparency, scalability, and ease of use.
Spine - Spine is 2D animation software designed specifically for software and game development.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.
Synfig Studio - Just another WordPress site
Godot Engine - Feature-packed 2D and 3D open source game engine.
Unity 2D - Unity 2D is a powerful and feature-rich game design solution built from the ground up to fulfill every basic and advanced need of the developer.