Xamarin Studio is a cross platform development tool, that you can use to develop iOS, Android, Windows and Mac OS X applications, in C#.
Xamarin Studio IDE is available for both Windows and Mac, and uses Mono .Net implementation. It also integrates with Visual Studio on Windows, allowing you to write native iOS and Android apps using Visual Studio.
There are several pricing options, but they do offer a starter edition which is free. Starter edition does allow you to deploy your apps to devices (for testing) and to deploy your apps to app stores, but is lacking some of the more advanced features and support.
I have just started playing with Xamarin Studio on my Mac, and gone through a couple of starter tutorials. Looks promising, so if you are looking to try new development tool, give this a go.