GTK
GTK+ is a multi-platform toolkit for creating graphical user interfaces.
18 Alternatives To GTK
Agar
Agar GUI
Avalonia
A multi-platform .NET UI framework
Chromely
Chromely is a lightweight alternative to Electron.NET, Electron for .NET/.NET Core developers.
Crazy Eddies GUI System
Crazy Eddie’s GUI (CEGUI) system is a graphical user interface C++ library.
Electron
Build cross platform desktop apps with web technologies
FLTK
Fast Light Toolkit - Fast Light Toolkit (FLTK)
Flutter by Google
Build beautiful native apps in record time í ½íº
Fox toolkit
fox-toolkit.org
JUCE
JUCE is a wide-ranging C++ class library for building rich cross-platform applications and plugins…
JavaFX
JavaFX is the next step in the evolution of Java as a rich client platform.
Lara
Lara Web Engine is a C# Web UI framework supporting web component development and reactive programming. - integrativesoft/lara
Nana C Library
Nana C++ Library - a modern C++ GUI library
PySide
Qt for Python - Qt Wiki
Qt
C++
Qt Creator
Qt Creator is a cross-platform C++, JavaScript and QML integrated development environment. It is the fastest, easiest and most fun experience a C++ developer could wish for.
Quasar Framework
SPA front-end on steroids.
Unity
The multiplatform game creation tools for everyone.
wxWidgets
wxWidgets: Cross-Platform GUI Library