Adding List Item to UI / Appending Html to control

<div id="header">
  <ul class="tabs">
    <li><a href="#"><span class="tab">ListItem1</span></a></li>
    <li><a href="#"><span class="tab">ListItem2</span></a></li>
  </ul>
</div>

To Add the new ListItem li using jQuery write as

$("#content ul li:last").append("<li><a href="#"><span class="tab">ListItem3</span></a></li>");