This is an example of the YUI AccordionView widget in action. This widget was created by Marco van Hylckama Vlieg. All code is licensed under the BSD license.
menu2 = new YAHOO.widget.AccordionView('mymenu2', {animationSpeed: '0.6', expandItem: 0, width: '600px', animate: true, effect: YAHOO.util.Easing.bounceOut});
Inside item 2 is a nested AccordionView with:
menu3 = new YAHOO.widget.AccordionView('mymenu3', {collapsible: false, width: '250px', expandItem: 2, animationSpeed: '0.5', animate: true, effect: YAHOO.util.Easing.easeNone});