Duality
a 2D Game Development Framework
...Duality is a modular 2D game engine that provides its own visual editor. It's highly extensible, written entirely in C# and backed by OpenGL. For a lot of game libraries, your code has the active role of setting everything up and managing it. These roles are reversed here: Your primary job is to define objects and specify their behaviors, Duality can handle all the rest. Drag files into the editor and drag assets into the game. Type C# code for game logic and engine extensions. Add bits of your code to objects in the game. Duality is being constantly developed since 2011, let's build some great games with it. ...