The new Dreamweaver CS3 comes with a Spry menu widget, which is probably the easiest to use. You could download a demo here: http://www.download.com/3000-2048_4-10701454.html

jQuery has some really cool plugins, which include menu related. http://docs.jquery.com/Plugins

Or use some not so new codes from dynamicdrive.com: http://dynamicdrive.com/dynamicindex1/index.html

make sure what you use is primary-browser compliant. if it isn't, make sure there is a fail-safe system of code/css that allows the person to still easily navigate the site. one additional thing: make sure that if your site is to be crawled by search engines, that the menus don't appear "blind" to them as that will reduce their crawling capabilities/performance.

onclick="Element.toggle('menu_id')"