Visual Studio for Mac is replacing Xamarin Studio
Xamarin Studio (was MonoDevelop) is an IDE primarily designed for C# and other .NET languages. Xamarin Studio enables developers to quickly write desktop and ASP.NET Web applications on Linux, Windows and OS X. It makes it easy for developers to port .NET applications created with Visual Studio to Linux and to maintain a single code base for all platforms.
For Mac and Windows, you can download Xamarin Studio which is a bundle of MonoDevelop along with Xamarin iOS/Android plugins and branding. Note that a Xamarin license is only required if you develop iOS/Android projects, all other project types are free without restrictions.