GacUI - GPU Accelerated C++ User Interface, with WYSIWYG developing tools, XML supports, built-in data binding and MVVM features. [Ms-PL] https://github.com/vczh-libraries/GacUI
GTK+ - A multi-platform toolkit for creating graphical user interfaces. [LGPL] http://www.gtk.org/
gtkmm - The official C++ interface for the popular GUI library GTK+. [LGPL] http://www.gtkmm.org/en/
libui - Simple and portable (but not inflexible) GUI library in C that uses the native GUI technologies of each platform it supports. [MIT] https://github.com/andlabs/libui
QwtPlot3D - A feature-rich Qt/OpenGL-based C++ programming library, providing essentially a bunch of 3D-widgets. [zlib] http://qwtplot3d.sourceforge.net/
Sciter - Sciter is an embeddable HTML/CSS/scripting engine aimed to be used as an UI layer of modern desktop applications. [Free/Commercial] http://sciter.com/
wxWidgets - A C++ library that lets developers create applications for Windows, Mac OS X, Linux and other platforms with a single code base. [Own LGPL] http://wxwidgets.org/