Qt 5

Agar   (Compare Project)

Mostly written in C

License: bsd

TnFOX   (Compare Project)

Mostly written in C++

License: lgpl

Qt 4   (Compare Project)

Mostly written in C++

License: gpl3, lgpl

Crazy Eddie's GUI System   (Compare Project)

License: mit

Ultimate++   (Compare Project)

Mostly written in C

License: BSD-3-Clause

OpenTK   (Compare Project)

Mostly written in C#

License: mit

jameica   (Compare Project)

Mostly written in Java

License: Unknown

Clutter Toolkit   (Compare Project)

Mostly written in C

License: lgpl21_or_later

Qt4 Bindings for Perl   (Compare Project)

Mostly written in Perl

License: gpl

Vexi   (Compare Project)

Mostly written in Java

License: apache_2, gpl, lgpl