Xamarin.Mac is a new foundation for building Cocoa applications on OSX using Mono.
Details about Xamarin.Mac are covered at http://xamarin.com/mac
There are plenty of Xamarin.Mac examples to get you started building your native Mac application.
A chat room to discuss Xamarin.Mac exists at https://gitter.im/xamarin/xamarin-macios.
We are using the firstname.lastname@example.org mailing list to discuss.