GTK, which stands for the Gimp ToolKit, is a library for creating graphical user interfaces. It is designed to be small and efficient, but still flexible enough to allow the programmer freedom in the interfaces created. GTK provides some unique features over standard widget libraries.
Zune is a free clone of the famous MUI (Magic User Interface) toolkit from the Amiga. It is primarily designed for C, and uses BOOPSI as an object-oriented layer. The MUI-apps developer should not care about "precise" graphic design (to let the user fully customize its application), and not care about resize (smart automatic layout algorithm). The MUI-apps user has maximum control over applications look and feel, and he can customize apps either globally or individually through a graphical prefs program.
Qt is a comprehensive, object-oriented development framework that enables development of high-performance, cross-platform rich-client and server-side applications. When you implement a program with Qt, you can run it on the X Window System (Unix/X11), Apple Mac OS X, and Microsoft Windows NT/9x/2000/XP by simply compiling the source code for the platform you want. Qt is the basis for the KDE desktop environment, and is also used in numerous commercial applications such as Google Earth, Skype for Linux, and Adobe Photoshop Elements.
The Xmt Library provides nine custom widgets and over 260 convenience functions designed to make Motif user interface development easier for beginning and advanced programmers. It provides an alternative tool chain that avoids the use of GUI builders. The essence of this approach is to program from the resources file using an extended vocabulary for automatic creation of the widget hierarchy, dynamic layout, and to provide a rapid prototype environment.
Tkinter is Python's de facto standard GUI (Graphical User Interface) package. It is a thin object-oriented layer on top of Tcl/Tk. To use Tkinter, you don't need to write Tcl code, but you will need to consult the Tk documentation and occasionally the Tcl documentation (since Tk's low-level event handling mechanism is considered part of Tcl).
KaiView is a GUI toolkit/framework for writing applications using the TOM programming language. Based on a vector-oriented drawing layer using libart and freetype2, it provides a widget layer with subject/view separation, forms loaded from XML files, live updates to the GUI, Unicode support, abstract manipulator classes, antialiased everything, and a lot more.