www.opencube.com
They have a free DHTML pop up menu script that as far as I can tell is very stable. And it seemed to me to be very easy to implement it. One little trick with it: you have to have their .js files in the same directory as your pages (if you have lots of folders, that means copying and pasting those files all over the place) *however* the file that you actually customize with your menus… THAT you can have in one place and reference it from anywhere on your site.
(I'm actually using this on a corporate website that I'm designing now… and I've seen this used on other corporate sites as well… www.bea.com is one of them.)