The MIT license is a permissive free software license which has a high-degree of license-compatibility. Developers embedding Mono, using Mono in DRM-protected solutions, or distributing their applications in app stores no longer need to acquire a commercial license in order to avoid the copyleft terms of earlier Mono releases. Please note that the Mono runtime and class libraries include some bits of code licensed by third parties under other open source licenses. The exact list is available here.
Xamarin is now included with Visual Studio. To develop iOS and Android apps with C#, download Visual Studio at xamarin.com/download.