JUCE is an open-source cross-platform C++ application framework
Cross-platform layout engine which implements Flexbox
Open source framework simplifying the creation of C++ applications
A text-based widget toolkit
C++ Frontend for the RenderMan Interface
Software libraries for effective development of desktop applications.
Object oriented frameworks - PowerBuilder, PowerServer & InfoMaker!
C++ Library for the Common Image Generator Interface
Development framework based on message driven provided
High performance C++ library for building Wayland compositors
The best library to work with .Net Assemblies in any programming lang.
The best library to work with .Net Assemblies in C/C++.
A framework for building Mobile cross-platform UI
Now maintained at https://code.google.com/p/lpi/
A minimalistic test framework written in C++.
PHP bindings for the cross-platform GUI library wxWidgets.
universal multi-platform light-weigth extensible scripting framework
Widgets library contains c++ classes of GUI widgets for WinAPI.