A lot of members here have used [dynamicdrive.com...] for this kind of navigation.
I personally take issue with this style of menu for usability reasons -- and where I've reworked the Information Architecture and navigation of a site that uses DHTML dropdown menus, I've been able to improve their stickiness by eliminating it.
The short version of my anti-DHTML menu rant is that it places too many choices on the page - and many people will make NO choice in such a situation. Some users love it - usually the more technicallly minded. But Mom and Pop AOL often run the other way.
However, I also recognize that sometimes the client wants what they want, and they will not be deterred.