The Giga Menu module enables a standard Drupal menu to be displayed as a Mega Menu.
Mega Menus are menus in which sub-items are displayed all at once, giving users access to the whole navigation menu tree, and thereby reducing the number of clicks required to navigate around a website.
Giga Menu module depends on hoverIntent module.
Mega Menu - at time of this writing Mega Menu module is a non-functioning alpha released in July 2010. Good project description, but we couldn't use the module.
Menu Minipanels - relies on Panels module, so if you're not using Panels use Giga Menu instead.
Development funded by Sage Pay.