Duality is a plugin based 2D game development framework using C# and OpenTK. One of its main features is the WYSIWYG editor, which lets you playtest and edit your game seamlessly within the integrated sandbox.
To learn more about the engine, please visit the project page.
Besides the github project overview and documentation, there is also a developer blog. If you want to contribute to the project, be it by active development, documentation or testing, you should pay a visit to the community forums. The same applies to any questions or suggestions - feedback is always welcome! However, bugs and feature requests that do not require further discussion should be filed in the issue tracking system, directly on github.