Qt
Xamarin
GTK
WompMobile
Oracle Mobile Application
wxWidgets
OutSystems
Mendix
SourceForge
GitHub
GitLab
BitBucket
openDesktop.org
Gitea
Launchpad.net
OSOR
SourceForgeXamarin - Create iOS, Android and Mac apps in C#
GitHub - Originally founded as a project to simplify sharing code, GitHub has grown into an application used by over a million people to store over two million code repositories, making GitHub the largest code host in the world.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
GitLab - Create, review and deploy code together with GitLab open source git repo management software | GitLab
WompMobile - WompMobile offers tow kind of functions โ first creating new mobile apps and secondly converting the websites into mobile applications.
BitBucket - Bitbucket is a free code hosting site for Mercurial and Git. Manage your development with a hosted wiki, issue tracker and source code.