QCustomPlot - Qt plotting widget without further dependencies. [GPLv3]
Qwt - Qt Widgets for Technical Applications. [Own based on LGPL]
QwtPlot3D - A feature-rich Qt/OpenGL-based C++ programming library, providing essentially a bunch of 3D-widgets. [zlib]
PDCurses - Public domain curses library with both source code and pre-compiled library available. [PublicDomain]
Sciter - Sciter is an embeddable HTML/CSS/scripting engine aimed to be used as an UI layer of modern desktop applications. [Free/Commercial]
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]
CImg - A small, open source, C++ toolkit for image processing. [Own LGPL or GPL]
CxImage - An image processing and conversion library to load, save, display, transform BMP, JPEG, GIF, PNG, TIFF, MNG, ICO, PCX, TGA, WMF, WBMP, JBG, J2K images. [zlib]
FreeImage - An open source library that supports popular graphics image formats and others as needed by today‘s multimedia applications. [GPL2 or GPL3]
GD - GD Graphics Library, famously used in PHP for image loading/manipulation & thumbnail generation. [custom permissive license, requires mention in user docs] website