Ordinarily in Drupal, in order to see the sub-items in a menu, you need to click the link of this item and wait for a full page load. After enabling DHTML Menu, clicking a menu link once will expand it to reveal its sub-items without reloading the page. A double-click on the item will load the page normally. Additionally, a cookie stores which menus are open and which are closed, so that the state of the menus remain consistent as you navigate around the site.
Note: In 6.x-3.0 and all later versions, DHTML Menu starts working as soon as the module is enabled. There are no blocks to replace and no block options to enable.
#D7CX: DHTML Menu has a D7 test version and will have a stable release for Drupal 7 the day Drupal 7 is released.