GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
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.
PyQt - Riverbank | Software | PyQt | What is PyQt?
wxWidgets - wxWidgets: Cross-Platform GUI Library
Avalonia - A multi-platform .NET UI framework
PySimpleGUI - A simple to use GUI that can create custom GUIs