You should check out YUI Menu. The Yahoo guys have made a very nice dropdown implementation that's very solid and well documented. There's also lots of examples.
http://developer.yahoo.com/yui/menu/
If you want a real-life example check out the components site: http://www.ezcomponents.org. There's also an article from Derick about using YUI Menu together with eZ Components: http://ezcomponents.org/resources/articles/article-2007-12-20-tree-yui
