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.
