ITK provides the image processing "engine", but has no front-end component. That is left up to other libraries.
The following libraries may be used to visualize the results from ITK image processing:
Using VTK with ITK
VTK is probably the most widely used visualisation library used with ITK. It is powerful, mature, easy to integrate, well documented, and plenty of examples exist. VTK is capable of working with both 2D and 3D data, and has very powerful and flexible processing and visualisation features.
Using OpenGL with ITK
OpenGL (homepage) is a low-level 3D rendering API, available on a wide variety of platforms. OpenGL can be used...
It is often used with a Scene Graph library, which provides a higher level scene-based interface to the developer. Some scene graph packages include: