Products of libufo.sourceforge.net
Multimedia \ Drawing and CAD
A C++ OpenGL GUI Toolkit. Visual output is customizable via CSS and style classes. Forms can be created via XUL (XML UI Language). There are builtin video drivers for GLX, WGL and SDL. Using dummy drivers, it is known to work with SDL, Glut, QT and GLFW..
Freeware download of UFO: Universal Form Objects 0.8.5, size