Xamarin is a cross-platform (iOS, Android, & Windows) mobile application development suite using C#; it runs on both Windows and OSX.
Microsoft bought Xamarin in February and have made it free for the majority of it's users and open sourced some major components. Xamarin used to run anywhere from $70 as a student through to $1000+ as a business.
From the blog -
"Xamarin will be in every edition of Visual Studio, including the widely-available Visual Studio Community Edition, which is free for individual developers, open source projects, academic research, education, and small professional teams. Develop and publish native apps for iOS and Android with C# or F# from directly within Visual Studio with no limits on app size.
For developers on the Mac, Xamarin Studio is now available as a benefit of your Visual Studio Professional or Enterprise subscription. Developers can use the newly-created Xamarin Studio Community Edition for free."