nav despleglable

<script type="text/javascript">
   function show(element) {
      if(element.value=='Other')
         document.getElementById('other').style.display='block';
      else document.getElementById('other').style.display='none';
   }
</script>
<form action="">
   <select name="some_name" onchange="javascript:show(this)">
      <option value="">--Please Choose--</option>
      <option value="Other">Other</option>
   </select>
   <div id="other" style="display:none">
      <input type="text" name="other_name">
   </div>
</form>