Code:
<ul id="dropdown">
    <li>
    <div id="menu_item">
        <div class='menu_links'>
            <a href='?page=15'>15</a>
        </div>
    </div>
    </li>
    <li>
    <div id="menu_item">
        <div class='menu_links'>
            <a href='?page=16'>16</a>
        </div>
    </div>
    </li>
    <ul>
        <li>
        <div id="submenu_item">
            <a href='?page=22'><img src="../images/arrow_link.png" align="top"/>16a</a>
        </div>
        </li>
        <li>
        <div id="submenu_item">
            <a href='?page=23'><img src="../images/arrow_link.png" align="top"/>16b</a>
        </div>
        </li>
        <li>
        <div id="submenu_item">
            <a href='?page=24'><img src="../images/arrow_link.png" align="top"/>16c</a>
        </div>
        </li>
        <li>
        <div id="submenu_item">
            <a href='?page=25'><img src="../images/arrow_link.png" align="top"/>16d</a>
        </div>
        </li>
    </ul>
    <li>
    <div id="menu_item">
        <div class='menu_links'>
            <a href='?page=17'>17</a>
        </div>
    </div>
    </li>
    <li>
    <div id="menu_item">
        <div class='menu_links'>
            <a href='?page=18'>18</a>
        </div>
    </div>
    </li>
    <li>
    <div id="menu_item">
        <div class='menu_links'>
            <a href='?page=19'>19</a>
        </div>
    </div>
    </li>
</ul>
</ul>

Code:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script type="text/javascript" src="http://ajax.googleapis.com/aja...queryui/1.5.3/jquery-ui.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){    
    $('#dropdown li').hover(
        function () {
            //show its submenu
            $(this).find('ul').show();
        }, 
        function () {
            //hide its submenu
            $(this).find('ul').hide();        
        })
});
</script>