menu (or its son
) but with some CSS 3 magic added to it. However, now that IE 6 is insignificant you can disregard the suckerfish part of the menu and just use the regular CSS.
It is really simple and the best way to learn is to look at and/or copy their source code (ideally with a code inspection/debugging tool which is usually already built into the browser). You have a list with nested sub lists, and you hide the sub lists and show them on
over the main list items. There is a little more to it in terms of styling but the functionality comes down to this.