FLTK!

Qt Creator is a cross-platform C++, JavaScript and QML integrated development environment which is part of the SDK for the Qt Framework.

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.

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.

wxWidgets is a C++ library that lets developers create applications for Windows, OS X, Linux and UNIX on 32-bit and 64-bit architectures as well as several mobile platforms including Windows Mobile, iPhone SDK and embedded GTK+. It has popular language bindings for Python, Perl, Ruby and many other languages. Unlike other cross-platform toolkits, wxWidgets gives its applications a truly native look and feel because it uses the platform's native API rather than emulating the GUI. It's also extensive, free, open-source and mature. Why not give it a try, like many others have? »

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++.

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

Crazy Eddie's GUI (CEGUI) system is a graphical user interface C++ library. It was designed particularly for the needs of video games, but the library is usable for non-game tasks, such as applications and tools. It is designed for user flexibility in look-and-feel, as well as being adaptable to the user's choice in tools and operating systems.

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.

PyGTK lets you to easily create programs with a graphical user interface using the Python programming language. The underlying GTK+ library provides all kind of visual elements and utilities for it and, if needed, you can develop full featured applications for the GNOME Desktop.