C# GUI in Linux/OSX

So far I've been doing C# developme开发者_高级运维nt with emacs + csharp-mode + mcs and it's been successfully emulating an IDE. Now I have to develop some Windows Forms application. Is there a tool for Linux like the simple GUI builder in VS? What about OS X?

Mono Develop http://monodevelop.com/ has support for building WinForms on Unix based systems.