If you are creating a custom themes options page or a Wordpress plugin which needs to get all the Wordpress menus in your site. This small function can be very useful, it will return all the Wordpress menus on your site in an array so you can process them as you want. This uses the Wordpress function wp_get_nav_menus to get all the nav_menu's on the site.
$menus = wp_get_nav_menus();