Xamarin.Android - Integrated environment for building not only native Android but iOS and Windows apps too.
wxWidgets - wxWidgets: Cross-Platform GUI Library
RAD Studio - RAD Studio 10.2 with Delphi Linux compiler is the fastest way to write, compile, package and deploy cross-platform native software applications. Learn more.
Electron - Build cross platform desktop apps with web technologies
Rider - Rider is a cross-platform .NET IDE based on the IntelliJ platform and ReSharper.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.