It is not only structured with correct HTML definition lists tags but also offers plenty customization (even more than shape5 relating to parent id items or open nodes), high compatibility and, above all, excellent email support (fastest ever seen!!). It also comes with documentation, so don't search any more, this is THE accordion menu.
Thanks Roland for such an excellent menu!!
tip: be aware to purge URL's in sh404SEF, before testing the menu ;)