Let me welcome you to the CocosSharp forum, the place where we discuss the future of the open source and cross platform CocosSharp stack.
CocosSharp offers a carefully crafted API that follows the .NET conventions and idioms and is delivered as a NuGet package in both full-stack access and Portable Class Library modes. We also ship with very comprehensive platform support. From Xamarin's favorites: Android, iOS and Mac to four variations of Windows and Windows Phone.
CocosSharp owes a lot to the great work that the Cocos2D-XNA team did (Jake and Gena at Totally Evil) and the many hackers that built Cocos2D-X and Cocos2D for Objective-C.
In the last couple of months we have shared this project with a few developers outside of Xamarin and we saw them contribute great pieces of technology like a C# implementation of Chipmunk (https://github.com/netonjm/ChipmunkSharp).