SoQt is a C++ GUI toolkit for using Coin with the Qt library. It can also be used on top of Open Inventor from SGI and TGS. The API is based on the InventorXt API originally from SGI.
Qt is a C++ toolkit for primarily the GUI parts of application development. Qt is a multi-platform library, available for X11-based systems (UNIX, Linux and *BSDs, for instance), MSWindows, Mac OS X and embedded systems.
For more information on the Qt GUI toolkit, see the web site for Troll Tech (makers of Qt): <http://www.trolltech.com>.
The corresponding documentation for Coin is located here.