ADVANCED JSMENU javascript is a script that adds a drop down menu, context menu, tree or tab controls to your page.
It's free, easy to use and extremely hight customizable with CSS. Each menu is implemented as plain HTML on the page, making the menu contents search engine friendly and potentially much easier to deploy and customize than menus where the contents are stored inside the script.
And to top things off, the drop down menu behaves intelligently in that if it's too close to the four corners of the browser window, it will reposition itself to avoid being obscured partly.
If your site is dynamic, this may be an easier script to deploy across the entire site.