Download Superfish from users.tpg.com.au/j_birch/plugins/superfish.
Superfish is used in the Aardvark theme used on this site. The menu bar across the top of the page was created in Aardvark from CSS based menus used in previous themes. Superfish adds a slight stability that makes mousing over the menus a delight.
The menus were developed years ago using HTML then updated to CSS as soon as CSS had the right facilities. CSS menus react too quickly and you often miss the right menu entry because the CSS menus flick away before you select the item you want. Superfish introduces a delay before the menu drop down elements disappear. With Superfish you get time to select the right entry.
Some Drupal themes are supplied with Superfish style menus built in. Aardvark is supplied with the original Superfish files untouched.
There is a Drupal add-on module, drupal.org/project/superfish, to add Superfish to existing menus if you use an older theme.