<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; }
/*--Notice the hover color is on the list item itself, not on the link. This is so it can stay highlighted even when hovering over the subnav--*/
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>