Qt - Powerful, flexible and easy to use, Qt will help you not only meet your tight deadline, but also reduce the maintainable code by an astonishing percentage.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
wxWidgets - wxWidgets: Cross-Platform GUI Library
PyQt - Riverbank | Software | PyQt | What is PyQt?
FLTK - Fast Light Toolkit - Fast Light Toolkit (FLTK)
PySimpleGUI - A simple to use GUI that can create custom GUIs