Dropdown Main (and not only main) Menu [Plugin or Widget]
1. shailan.com
2. designchemical.com

In more details:

1. shailan.com
---------------
Dropdown Menu Widget, 2.8.1, updated 2011.12.14, WP-3.3.1 compatible.
http://shailan.com/wordpress/plugins/dropdown-menu/
- dropdown menu information

http://shailan.com/wordpress/plugins/dropdown-menu/menu-themes/
- themes screenshots

Install, activate, change settings
and add
<?php shailan_dropdown_menu(); ?>
in the header. 

Other widgets-plugins there:
- Superfish Dropdown Menu.
- Subpages Extended Widget.
- Dropdown Multi Widget.

2. designchemical.com
-----------------------
JQuery Mega Menu Widget 1.3.8

http://www.designchemical.com/blog/index.php/wordpress-plugins/wordpress-plugin-jquery-drop-down-mega-menu-widget/
- general information

http://www.designchemical.com/lab/demo-wordpress-jquery-mega-menu-plugin/
- demo, revealing various settings

http://www.designchemical.com/lab/demo-custom-mega-menu-styling/
- demo with custom styling

Other widget-plugins of designchemical.com:

http://www.designchemical.com/blog/index.php/wordpress-plugins/wordpress-plugin-jquery-vertical-mega-menu-widget/
- JQuery Vertical Mega Menu Widget

The plugin may require a special new widget place to add to the theme. Here is the FAQ how to do it:
http://www.designchemical.com/blog/index.php/wordpress-tips/wordpress-tutorial-adding-a-widget-area-to-your-theme-files/

INSTRUCTION
Creating a new widget area.

Within php tags in functions.php:

if ( function_exists('register_sidebar') ){
register_sidebar(array(
'name' => 'my_mega_menu',
'before_widget' => '<div id="my-mega-menu-widget">',
'after_widget' => '</div>',
'before_title' => '',
'after_title' => '',
));
}

then...

Give the widget area a name – e.g. my_mega_menu and in the before/after options you can place code which you wish to appear wrapped around either the widget itself or the title.

Since we are wanting to use one of the menu widget themes, which come with plugins we dont want to wrap it in the standard widget tags, which may interfere with the CSS.

For this example we have just wrapped it in a div tag with the id – my-mega-menu-widget.

2. Add The Widget Code To Your Theme
Now we can add the code that will call the widget to the correct location in our theme files. Since we are creating a horizontal mega menu we obviously want to display this in the header.

In your WordPress themes editor open the header.php file.

Find the location where you want to place the menu and add the following code to the header.php file:

<?php /* Widgetized sidebar */
if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar('my_mega_menu') ) : ?><?php endif; ?>

Make sure that you change the “my_mega_menu” text to be the same as the name you gave to your widget area in the functions file.

Save the changes to the header.php file

If you now go to the widget area in WordPress admin you should see a new widget area in the right hand column with the name of your widget.

You can now add your mega menu, accordion menu or any other widget to this area.

Source:
http://www.designchemical.com/blog/index.php/wordpress-tips/wordpress-tutorial-adding-a-widget-area-to-your-theme-files/