Nice Menus enables drop-down/right/left expandable menus. It uses only CSS for most browsers, with minimal Javascript for IE6. (Version 2 uses the Superfish jQuery plugin for all browsers, with an option to disable JS, and falls back to CSS-only for browsers that can handle it.)
Three styles/types of menus are currently possible: horizontal, menus drop down; vertical, menus fly to the left; vertical, menus fly to the right. There is a handbook page that provides a list of sites that use Nice menus.
Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution 3.0 Unported License.