Xamarin.Forms: Native Android, iOS, and Windows apps in C#
With Xamarin.Forms, you can build cross-platform native UIs with one shared codebase. Learn how to build a shared UI completely in XAML that targets iOS, Android, and Windows. Take advantage of the .NET Framework to build a shared back end of business logic for the apps. Additionally, learn about built-in two-way data binding, an advanced cross-platform animation system, support for custom controls, and other powerful features to help you build the best apps in the least amount of time.
To get started and download the bits: http://xamarin.com/download