Xamarin has released Xamarin Mac which adds Mac support to the existing iOS and Android compilers from the company:
MonoTouch: apps for iPhone and iPad using the MonoDevelop IDE on the Mac Mono for Android: apps for Android using either Visual Studio or MonoDevelop Xamarin.Mac: apps for Mac OS X using MonoDevelop on the Mac...continue reading Xamarin brings C# to development of apps for the Mac App Store