JUCE is described as 'has hundreds of classes covering a vast range of tasks from high-level user-interface handling right down to low-level collections, networking, strings, etc' and is an app in the Development category. There are more than 10 alternatives to JUCE for a variety of platforms, including Mac, Linux, Windows, Android and iPhone. The best alternative is Qt, which is both free and Open Source. Other great apps like JUCE are Qt Creator (Free, Open Source), GTK+ (Free, Open Source), wxWidgets (Free, Open Source) and Avalonia (Free, Open Source).
RtAudio is a set of C++ classes that provide a common API for realtime audio input/output across Linux, Macintosh OS-X and Windows operating systems. RtAudio significantly simplifies the process of interacting with computer audio hardware.
Nana C++ Library takes aim at easy-to-use and portable library, it provides a GUI framework and threads for easy programming with modern C++ methods, such as traits, metaprogramming and other template technologies. http://sourceforge.net/p/nanapro/blog
Crazy Eddie's GUI (CEGUI) system is a graphical user interface C++ library. It was designed particularly for the needs of video games, but the library is usable for non-game tasks, such as applications and tools.
Photon Micro GUI is a modern C++ (esp. C++14 and C++17) UI library. Featuring resolution independence and HDPI support, with modularity as another focal point, the library is made for general purpose use while also covering everything needed for audio plugin development.
KFR is built for developers who need a powerful toolkit for audio and DSP applications. The framework is packed with ready-to-use C++ classes and functions for various DSP tasks from high-quality filtering to small helpers to improve development speed.