[tiernano]

Mono is the framework (like .Net) and Xamarin is the tooling (like Visual Studio). Framework is free (essentially) and the tooling is like VS (starter is free, pro is not so free but has more tools)

[jchannon]

OmniSharp will help http://omnisharp.net

[carussell]

Xamarin Studio is just a rebranded MonoDevelop with a few proprietary Xamarin supplements.