CEGUI
Since 2003, Crazy Eddie’s GUI has been under development. Version 0.87 is currently underway so version 1.0 is expected by 2020. CEGUI can function on Mac, Windows and Linux, and supports both 32- and 64-bit.It is popular, and you can either crib one of the existing renderer backends or write your own targeting whatever OpenGL-backed API you want.
