Xamarin

Xamarin is a San Francisco, California based software company created in May 2011 by the engineers that created Mono, MonoTouch and Mono for Android which are cross-platform implementations of the Common Language Infrastructure (CLI) and Common Language Specifications (often called Microsoft .NET).

With a C# shared codebase, developers can use Xamarin to write native iOS, Android, and Windows apps with native user interfaces and share code across multiple platforms and has over 505,000 developers in more than 120 countries around the world as of February 2014.