ADxMenu - multilevel menu built from nested HTML lists - is my pride and joy. I began working on it last year, always implementing the good stuff I learned on the web standards road. In its current incarnation, I believe it’s best of the breed. There are other menus out there (using the very similar HTML base), but most of them can, at most, measure up to ADxMenu standard.
With that said, I want to repeat what was my target from day 1. It’s valid, standard-compliant web sites. If you have an old-school web site with nested tables and no DOCTYPE - do not bother; you have other things to conquer first. On the other hand, if you are a CSS newbie/inter/pro - you are welcomed. You will need good CSS skills to create attractive menus though; I did my best to give you a good starting point, for 4 different variants of the menu, but the final look is up to you.
There are many old-school menus with large scripts and over-bloated markup. Those few that are good and light are not free. ADxMenu3 is free to use on any kind of site, as long as you give proper attribution.