Graphical User Interfaces (GUIs) in Python

For simple command line programs

Gooey - Turn (almost) any Python command line program into a full GUI application

GUI libraries

TkInter - Python's de-facto standard GUI available in the standard library
PySimpleGUI - Transforms tkinter, Qt, Remi, WxPython into portable people-friendly Pythonic interfaces. Easy for beginners, not really recommended for production ready app.
Kivy - Cross-platform GUI library supporting both desktop operating systems (Windows, macOS, Linux) and mobile devices (Android, iOS).
PyQt5, QtPy, PySide - There are Python bindings available for the Qt toolkit (using either PyQt or PySide). PyQt is currently more mature than PySide, but you must buy a PyQt license if you want to write proprietary applications. PySide is free for all applications.
DearPyGUI - Has nice documentation, good tutorial made by dev himself, loaded with features but frequent updates change the functionality sometimes. Built with Dear ImGui.
WX-Python - Hasn't been updated in a while.
Toga - Still in beta.

Other

Streamlit - Makes it easy to build beautiful apps for machine learning.