Elementary (EFL)

Qt 5   (Compare Project)

Mostly written in C++

License: Commercial, GNU_Free_Documentation_License, gpl3, lgpl21

EFL (Enlightenment Foundation Libraries)   (Compare Project)

License: bsd_2clause_ish, gpl, lgpl21_or_later, zlib_libpng

GTK   (Compare Project)

Mostly written in C

License: lgpl21_or_later

Qt 4   (Compare Project)

Mostly written in C++

License: gpl3, lgpl

Clutter Toolkit   (Compare Project)

Mostly written in C

License: lgpl21_or_later

OpenTK   (Compare Project)

Mostly written in C#

License: mit

Crazy Eddie's GUI System   (Compare Project)

License: mit

TnFOX   (Compare Project)

Mostly written in C++

License: lgpl

Pygame   (Compare Project)

Mostly written in Python

License: lgpl

GtkGLExt   (Compare Project)

License: lgpl