A free, cross-platform C++ GUI library. It features an event messaging and targeting scheme with a very clean, factored, OS-abstracted design. [Open Source, LGPL]

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.

A C++ cross-platform rapid application development suite. It includes a set of libraries (GUI, SQL, etc..), and an integrated development environment with GUI designer, debugger, etc.

A C++ wrapper to GTK+. It base on VDK, the Visual Development Kit. [Open source, GPL]

A C++ library for developing Windows applications and UI components. It extends ATL (Active Template Library) and provides a set of classes for controls, dialogs, frame windows, GDI objects. [Open Source]

Wt (pronounced 'witty') is a C++ library and application server for developing and deploying web applications. The API is widget-centric, and inspired by existing C++ Graphical User Interface (GUI) APIs. To the developer, it offers complete abstraction of any web-specific implementation details. [Open Source, GPL]