WordPress menu plugins can be very useful for creating mega menus that are very well organized. Most WordPress menu plugins are based on jQuery menu scripts which are known for their great usability. You can add these to your theme by using a code but it is much easier to simply install a WordPress plugin.
UberMenu Conditionals adds the option to conditionally display or hide any item in your UberMenu. It adds a new option to each menu item, where you can select which condition must be met via a select box.
Stickr WP is a sticky side panel menu WordPress plugin. It has an intuitively easy administration panel, so you don’t have to know the coding behind it.
This plugin adds an enhanced Navigation Menu widget. It offers many options which could be set to customize the output of the custom menu through the widget.
Dropdown Menu widget adds a beautiful, CSS only dropdown menu, listing pages, categories of your blog. You can also turn your WordPress navigation menu into a beatiful dropdown menu using this plugin.
The AllWebMenus WordPress Menu add-in & plugin combination is designed to transfer information from your blog into the AllWebMenus application and then create stylish, feature-rich navigation menus based on those posts, pages, etc.
Submenus enhance the WordPress 3.x navigation system. When you have a website based on WordPress with a lot of pages, but you need a custom menu, it can be tedious to have to add a menu entry for each page created. Submenu just does it automatically.
This WP plugin adds a CMS-like tree overview of all your pages and custom posts to WordPress. Within this you can edit pages, view pages, add pages, search pages, and drag and drop pages to rearrange the order.
Page Tree gives you a much-needed overview of your pages in the admininstration panel using a common expand/collapse menu, which lets you navigate your page structure like a folder tree in Windows Explorer.
The Admin Menu Tree Page View WordPress plugin adds a page tree to all your pages and it is directly accessible in the admin menu. This way all your pages will be available within just one click, no matter where you are in the admin area.
The WordPress 3 Menu Management System is excellent. The Menu Management Enhancer for WordPress aims to improve the menu management user experience.