xhtml & css

<ul id="topnav">
    <li><a href="#">Link</a></li>
    <li>
        <a href="#">Link</a>
        <!--Subnav Starts Here-->
        <span>
            <a href="#">Subnav Link</a> |
            <a href="#">Subnav Link</a> |
            <a href="#">Subnav Link</a>
        </span>
        <!--Subnav Ends Here-->
    </li>
    <li><a href="#">Link</a></li>
</ul>

ul#topnav {
	margin: 0; padding: 0;
	float: left;
	width: 970px;
	list-style: none;
	position: relative; /*--Set relative positioning on the unordered list itself - not on the list item--*/
	font-size: 1.2em;
	background: url(topnav_stretch.gif) repeat-x;
}
ul#topnav li {
	float: left;
	margin: 0; padding: 0;
	border-right: 1px solid #555; /*--Divider for each parent level links--*/
}
ul#topnav li a {
	padding: 10px 15px;
	display: block;
	color: #f0f0f0;
	text-decoration: none;
}
ul#topnav li:hover { background: #1376c9 url(topnav_active.gif) repeat-x; }
ul#topnav li span {
	float: left;
	padding: 15px 0;
	position: absolute;
	left: 0; top:35px;
	display: none; /*--Hide by default--*/
	width: 970px;
	background: #1376c9;
	color: #fff;
	/*--Bottom right rounded corner--*/
	-moz-border-radius-bottomright: 5px;
	-khtml-border-radius-bottomright: 5px;
	-webkit-border-bottom-right-radius: 5px;
	/*--Bottom left rounded corner--*/
	-moz-border-radius-bottomleft: 5px;
	-khtml-border-radius-bottomleft: 5px;
	-webkit-border-bottom-left-radius: 5px;
}
ul#topnav li:hover span { display: block; } /*--Show subnav on hover--*/
ul#topnav li span a { display: inline; } /*--Since we declared a link style on the parent list link, we will correct it back to its original state--*/
ul#topnav li span a:hover {text-decoration: underline;}

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3/jquery.min.js"></script>

<script type="text/javascript">
$(document).ready(function() {
 
	$("ul#topnav li").hover(function() { //Hover over event on list item
		$(this).css({ 'background' : '#1376c9 url(topnav_active.gif) repeat-x'}); //Add background color and image on hovered list item
		$(this).find("span").show(); //Show the subnav
	} , function() { //on hover out...
		$(this).css({ 'background' : 'none'}); //Ditch the background
		$(this).find("span").hide(); //Hide the subnav
	});
 
});
</script>