Xamarin Studio is better than MonoDevelop and (at least this is what they claim) free to use for Desktop (Window and Mac) applications http://www.monodevelop.com/download/ :

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.