
Most of the Windows.Forms API will work on Mono, however some applications (and especially third party controls) occasionally bypass the API and P/Invoke straight to the Win32 API. Net implementation is a binding to the Win32 toolkit, the Mono implementation is written in C# to allow it to work on multiple platforms. As a popular toolkit used by millions of Windows developers (especially for internal enterprise applications), the Mono project decided to produce a compatible implementation (Winforms) to allow these developers to easily port their applications to run on Linux and other Mono platforms.
Windows.Forms is a binding developed by Microsoft to the Win32 toolkit. The MonoMac APIs replaced the old CocoaSharp binding, which is now deprecated. MonoMac allows developers to access the whole range of MacOS X APIs from C#, it is not limited to the AppKit GUI APIs. Net/Mono developers that want to allow their users to have a native Mac OS X application experience.
Gtk+ apps run like foreign applications on MacOS X.
The API is fairly stable at this point, and syntactic sugar is being added to improve it. A Win32 port is available, with native look on Windows XP. Applications integrate with the Gnome Desktop. Layout engine ideal for handling internationalized environments, and adapts to font-size without breaking applications. Good support for accessibility through its Gtk+ heritage. In general, GTK# applications are written using MonoDevelop, which provides a visual designer for creating GTK# GUIs. It is in active development by the Mono project, and there are various real-world applications available that use it ( Banshee, F-Spot, Beagle, MonoDevelop). The toolkit is written in C for speed and compatibility, while the GTK# binding provides an easy to use, object oriented API for managed use. If you need help to know which one is the best for your project, please have a look at Choosing a GUI Toolkit. This page contains a list with a couple of known toolkits for Graphical User Interfaces that you can use in your Mono software.