WordPress.org

This plugin is a WordPress Google Summer of Code 2014 project, soon to be a WordPress core feature-plugin. See the updates on Make WordPress Core for more information.

The Menu Customizer adds custom menu management to the Customizer. It is not fully functional and in alpha development until further notice; please don't try to run it on a production site. The plugin requires WordPress 4.0 and PHP 5.3 or higher. It is mostly feature-complete and ready for testing, although there are known issues around adding new menus (requires some core work) and working with submenus (try the reorder mode, not drag-and-drop for now). Adding menus items could use some work as well, and is likely to have issues on sites with a large number of posts.

If you're interested in contributing to this project, stay tuned to http://make.wordpress.org/core for updates.

Core Patches

Several improvements to the Customizer are also in the works as a part of the GSoC portion of this project, in the form of core patches (for example, the Panels API). See Customizer Improvements in 4.0, and an upcomming Customizer Roadmap for details.