Webfish Dropdown Menu plugin by Tobias Nyholm gives the WordPress site admin the alternative to add a dropdown menu. This plugin does not require any programming or adding lines in the source code. Simply just activate the plugin, rename a menu, then customize it from the admin panel.
Webfish Dropdown Menu uses CSS to display the menu. There is no javascript included.
This plugin does not support the following at the current version: * vertical menus * background images for the submenu * multiple dropdown menus
More info: http://www.webfish.se/wp/plugins/webfish-dropdown-menu