Menu Toggle is a very small, simple module that lets you toggle the visibility of expanded menu items. By default when a Drupal menu is set to 'Expanded', the children of that menu item are listed below it. Some menus have very long lists of items however, and it can be inconvenient to navigate. With Menu Toggle you can force expanded menu items to show up only when you click the parent menu item. This will save a lot of vertical real estate on your website. Since the menu tree is already loaded during the bootstrap process, there is no AJAX involved - just a simple toggling of the display using a bit of jQuery. You also have the option of displaying the expanded menu items by default. Items in the active menu tree are always expanded.
Note that in order for a menu item to be togglable, the parent menu item must be set to 'Expanded' in Drupal's administration interface. By doing this you have complete control over which menu items can be toggled and which can't.
If you're looking for a more robust solution, check out DHTML Menu.
This module was co-developed by Venkat Dinevahi (vendiddy)