Activemenus makes the standard Drupal navigation menu expandable. Clickable, highlighted regions are added to the menu. When a collapsed menu item is expanded, the child menu items are loaded dynamically through AJAX.
The module DHTML Menu handles expandable menus too, but it does not use AJAX and the entire menu tree is loaded in the HTML, so it may cause some issues with very large menus.
This version allows to choose the menus on which the AJAX behavior should be activated and supports horizontal menus.
Port to Drupal 7 just began, thanks to alexh. See for helping.