SharpDevelop is an IDE for Windows. It supports C#, Visual Basic, Boo, and F#.
SharpDevelop can be used to compile and run applications with mono, something that Visual Studio can't do.
SharpDevelop 2.x includes mono in its default installation. Just create a new Mono solution and select the Mono framework you need (1.1 or 2.0) from the Project Options. 
SharpDevelop 3.0 beta and later only distribute Mono in source code form, so you'll have to compile it (very simple and quick) before being able to use mono with SD 3.0. What does SD 3.0 offer that 2.0 doesn't? Mainly C# 3.0 code completion.
Unfortunately, SharpDevelop doesn't have a GTKSharp designer like MonoDevelop does. That doesn't mean you can't create GTK#; you can do so by adding a reference to the library, but you won't have a designer to drag n drop controls onto a form.