ITK is an image processing engine. It does not provide any graphical interface or visualisation facilities. There are many GUI toolkits available, many of which are cross-platform. All are C++, unless otherwise noted. Here is a summary, while sections below discuss each project in more detail.
|FLTK - Fast Light Toolkit||http://www.fltk.org/||LGPL||Linux Windows Solaris Mac|
|Qt||http://www.trolltech.com/||QPL||Linux Windows Solaris Mac|
|Gtk+||http://www.gtk.org/||LGPL||Linux Windows Solaris|
|wxWidgets||http://www.wxwidgets.org/||LGPL||Linux Windows Solaris Mac|
A Note on Cross-Platform Support
There are many toolkits available that deliver cross-platform support. By using one of these libraries and making your software portable, you open up the potential user base to many more users. Users have more choice, are free to use their preferred platform, and your software can be expanded in the future to take advantage of other platforms, giving it wider appeal and greater longevity. Writing portable software is not hard, and benefits everyone in the long run. So please consider this when choosing your toolset.
The Fast Light Toolkit. A small, fast, lightweight toolkit. Comes with an interface builder (called Fluid). Integrates very well with OpenGL and VTK. Used by many of the example InsightApplications.
A very popular cross-platform toolkit and application framework. Includes application framework classes. Currently ITK does not provide a simple binding to this toolkit. However you can use wxVTK since ITK allows you to go back forth beween VTK.
The Gimp toolkit. The native toolkit for Gnome under Linux, it is now available for Win32 also. The original Gtk+ library is written in C, but C++ bindings exist in the form of Gtk-mm.
A comprehensive cross-platform class library, including GUI toolkit, application framework and utilities. QtDesigner is available. Not free for Win32.
The GUI toolkit that comes with Visual Studio, MFC is basically a thin class wrapper around the Win32 API. Note that this library limits you to only supporting Windows.
This is Apple's next-generation interface library for Mac OS X, written in Objective-C, based on the NextStep platform. A Free/Open project called GnuStep aims to replicate Cocoa for POSIX platforms (such as Linux).