Menu Parent is a module that allows menus to be mapped as children of other menus.
Why do I need this module? Can't menu items be nested normally?
Yes they can. But if you need the ability to allow certain users or roles to administer sections of the menu tree, the easiest thing to do is use the Menu Admin Per Menu module.
Adding this module then allows you to nest entire menus. It's otherwise fairly useless without the Menu Admin Per Menu module, although it theoretically would still work.
This module is seeking a new maintainer: I no longer work with Drupal on a day to day basis, and am unsure if this module is still relevant, if it is, and someone would like a D7 port, I'll gladly turn it over to a new maintainer.