Just recently, I used the Yahoo User Interface library to do that - http://developer.yahoo.com/yui/. It has lots useful DHTML/Javascript based UI widgets. The menu one is http://developer.yahoo.com/yui/menu/. The default look is very Yahoo-ish, but all of it is customizable with CSS. I can sent you my code if you want (mine doesn't have arrows, but does expand & collaspe on hover)
no subject