MonoMac
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
Samples
There are plenty of Xamarin.Mac examples to get you started building your native Mac application.
Community
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 [email protected] mailing list to discuss.
Obtaining MonoMac
Xamarin.Mac is available as part of Xamarin Studio or Visual Studio to download your package.