EFL (Enlightenment Foundation Libraries)

Qt 5   (Compare Project)

Mostly written in C++

License: Commercial, GNU_Free_Documentation_License, gpl3, lgpl21

Elementary (EFL)   (Compare Project)

License: lgpl21_or_later

Qt 4   (Compare Project)

Mostly written in C++

License: gpl3, lgpl

Ultimate++   (Compare Project)

Mostly written in C

License: BSD-3-Clause

Fast Light Tool Kit   (Compare Project)

Mostly written in C++

License: lgpl

Crazy Eddie's GUI System   (Compare Project)

Mostly written in C++

License: mit

GTK   (Compare Project)

Mostly written in C

License: lgpl21_or_later

PixelLight   (Compare Project)

Mostly written in C++

License: mit

Vexi   (Compare Project)

Mostly written in Java

License: apache_2, gpl, lgpl

AllBinary Platform   (Compare Project)

Mostly written in Java

License: Unknown