<script language="JavaScript">
<!-- 
function goToURL(form)
  {

    var myindex=form.dropdownmenu.selectedIndex
    if(!myindex=="")
      {
        window.location.href=form.dropdownmenu.options[myindex].value;

        // This is an alternative method you can use
        // var frameTarget = "_new";
        // window.open(form.dropdownmenu.options[myindex].value,frameTarget,"");
      }
}
//-->
</script>
<form name="dropDownForm">

  <select name="dropdownmenu" size=1 onChange="goToURL(this.form)">
    <option selected value="">Заголовок меню
    <option value=>Пункт1
    <option value=>Пункт2
    <option value=>Пункт3
    <option value=>Пункт4
    <option value=>Пункт5
  </select>
</form>