There are several cross-platform libraries for creating GUIs (graphical user interfaces), and the best one for you will depend on your specific needs and preferences. Here are a few popular options:

  1. Qt: Qt is a widely-used C++ framework for developing cross-platform applications, including GUIs. It provides a large set of widgets, layout managers, and tools for creating graphical interfaces. Qt is available under both open-source and commercial licenses, and it supports multiple operating systems, including Windows, Linux, and macOS.

  2. wxWidgets: wxWidgets is a C++ library for developing cross-platform GUI applications. It provides a set of widgets and tools for creating graphical interfaces, and it supports multiple operating systems, including Windows, Linux, macOS, and others. wxWidgets is available under an open-source license.

  3. GTK+: GTK+ is a popular C-based library for developing cross-platform applications, including GUIs. It provides a set of widgets and tools for creating graphical interfaces, and it supports multiple operating systems, including Windows, Linux, and macOS. GTK+ is available under an open-source license.

  4. JavaFX: JavaFX is a cross-platform GUI toolkit for developing Java-based applications. It provides a set of UI controls and layout managers for creating graphical interfaces, and it supports multiple operating systems, including Windows, Linux, and macOS.

  5. Electron: Electron is a popular framework for building cross-platform desktop applications using web technologies such as HTML, CSS, and JavaScript. It provides a browser window as the main application interface and allows for easy integration with web-based tools and services.

These are just a few of the many options available for creating cross-platform GUIs. Each library has its own strengths and weaknesses, so it's important to evaluate your specific needs and requirements before choosing one.