[devote]

<style type="text/css">
#menu li ul {
    display:none;
}
</style>
<ul id="menu">
    <li><a href="#" onclick="openMenu(this);return false">menu 1</a>
    <ul>
        <li><a href="#">подменю1</a></li>
        <li><a href="#">подменю2</a></li>
        <li><a href="#">подменю3</a></li>
    </ul>
    </li>
</ul>
<script type="text/javascript">
function openMenu(node) {
    var style,
        subMenu = node.parentNode.getElementsByTagName("ul")[0];

    if ( subMenu.ownerDocument && subMenu.ownerDocument.defaultView ) {
        style = subMenu.ownerDocument.defaultView.getComputedStyle( subMenu, null );
    } else if ( subMenu.currentStyle ) {
        style = subMenu.currentStyle;
    }

    style.display == "none" ? subMenu.style.display = "block" : subMenu.style.display = "none";
}
</script>