Xamarin.Mac
Xamarin.Mac is a new foundation for building Cocoa applications on macOS using Mono.
Details about Xamarin.Mac are covered at http://xamarin.com/mac
Samples
There are plenty of Xamarin.Mac examples to get you started building your native Mac application.
Community
A chat room to discuss Xamarin.Mac exists at https://gitter.im/xamarin/xamarin-macios.
Obtaining Xamarin.Mac
Xamarin.Mac is available as part of Xamarin Studio or Visual Studio to download your package.