This plugin creates vertical mega menus for categories, allowing users to view available menu options for each top-level menu item.

There are several plugin options available for customising the mega menu:

Choose animation effect for the flyout menu – show/hide, fade in or slide
Set animation speed
Display the flyout mega menu on the left or right depending on the menu position

Demo
http://demo15.proopencart.com/index.php?route=product/category&path=20