Commonly used tools and frameworks for cross-platform GUI development include:

1. Qt: A comprehensive C++ framework for building GUI applications.

2. wxWidgets: A C++ library for creating native-looking GUIs.

3. Flutter: A Dart-based framework for building natively compiled applications.

4. Electron: A JavaScript framework for building cross-platform desktop apps.

5. Swing and JavaFX: Java libraries for creating GUI applications.

6. Xamarin.Forms: A C# framework for building cross-platform mobile apps.

7. GTK+: A C library for creating GUI applications.

8. Enlightenment Foundation Libraries (EFL): C libraries for building GUI applications.

9. SDL (Simple DirectMedia Layer): A C library for building games and GUI applications.

10. PyQt and PySide: Python bindings for Qt.

11. Kivy: An open-source Python library for rapid development of GUI applications.

12. Tkinter: A Python library for creating GUI applications.

These tools and frameworks enable developers to build GUI applications that run on multiple platforms, including Windows, macOS, Linux, Android, and iOS, with minimal platform-specific modifications.