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 MonoMac exists on the IRC server at irc.gnome.org on the channel #monomac. You can access it on the web as well.
We are using the firstname.lastname@example.org mailing list to discuss.