Fox toolkit!

PyQt is a set of Python bindings for Nokia's Qt application framework and runs on all platforms supported by Qt including Windows, MacOS/X and Linux. There are two sets of bindings: PyQt v4 supports Qt v4; and the older PyQt v3 supports Qt v3 and earlier. The bindings are implemented as a set of Python modules and contain over 300 classes and over 6,000 functions and methods.

JavaFX is the next step in the evolution of Java as a rich client platform. It is designed to provide a lightweight, hardware-accelerated Java UI platform for enterprise business applications. With JavaFX, developers can preserve existing investments by reusing Java libraries in their applications. They can even access native system capabilities, or seamlessly connect to server-based middleware applications.

The PySide project provides LGPL-licensed Python bindings for the Qt cross-platform application and UI framework. PySide Qt bindings allow both free open source and proprietary software development and ultimately aim to support all of the platforms as Qt itself.

wxPython is a GUI toolkit for the Python programming language. It allows Python programmers to create programs with a robust, highly functional graphical user interface, simply and easily. It is implemented as a Python extension module (native code) that wraps the popular wxWidgets cross platform GUI library, which is written in C++.

JUCE has hundreds of classes covering a vast range of tasks from high-level user-interface handling right down to low-level collections, networking, strings, etc. Supported platforms are OSX, Windows, Linux, iOS and Android, and the Introjucer project management tool makes it a breeze to create and maintain cross-platform projects.

Nana C++ Library takes aim at easy-to-use and portable library, it provides a GUI framework and threads for easy programming with modern C++ methods, such as traits, metaprogramming and other template technologies. http://sourceforge.net/p/nanapro/blog

The Fast, Light Toolkit (FLTK, pronounced fulltick) is a cross-platform graphical user interface (GUI) library developed by Bill Spitzak and others. Made to accommodate 3D graphics programming, it has an interface to OpenGL, but it is also suitable for general GUI programming.

SWT is an open source widget toolkit for Java designed to provide efficient, portable access to the user-interface facilities of the operating systems on which it is implemented.

wxSVG is C++ library to create, manipulate and render SVG files.

IUP is a multi-platform toolkit for building graphical user interfaces. It offers a simple API in three basic languages: C, Lua and LED. IUP's purpose is to allow a program source code to be compiled in different systems without any modification.

gtkmm is the official C++ interface for the popular GUI library GTK+. Highlights include typesafe callbacks, and a comprehensive set of widgets that are easily extensible via inheritance. You can create user interfaces either in code or with the Glade User Interface designer, using Gtk::Builder. There's extensive documentation, including API reference and a tutorial.