Net-Tuts+ offers a really good tutorial on how to create a dynamic drop down menu using JQuery and CSS. Here is a little blurb about the tutorial:

We will use jQuery to create a different multi-layered horizontal navigation system that is still intuitive enough for anyone to use for the first time.

To view the tutorial follow this link: http://net.tutsplus.com/tutorials/javascript-ajax/a-different-top-navigation/