Xamarin

Free. Cross-platform. Open source.

An app platform for building Android and iOS apps with .NET and C#.

Snippet from Wikipedia: Xamarin

Xamarin is a Microsoft-owned San Francisco-based software company founded in May 2011 by the engineers that created Mono, Xamarin.Android (formerly Mono for Android) and Xamarin.iOS (formerly MonoTouch), 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 tools to write native Android, iOS, and Windows apps with native user interfaces and share code across multiple platforms, including Windows, macOS, and Linux. According to Xamarin, over 1.4 million developers were using Xamarin's products in 120 countries around the world as of April 2017.

On February 24, 2016, Microsoft announced it had signed a definitive agreement to acquire Xamarin.