[lord2kim]

function showWork(name){
   var link = this.children[0];
   link.className = 'active';
}

<ul><li class="leftmenu" onclick="showWork('gtl', this)">
    <a class="passive" href="#">GTL</a>
</li>
<li class="leftmenu" onclick="showWork('sotrudnichestvo', this)">
    <a class="passive" href="#">Сотрудничество</a>
</li>
</ul>
<script>
function showWork(name, href){
    /*var previous=document.getElementsByClassName('active');
        for (var i = 0; i < previous.length; i++) {
            previous[i].className="passive";
        }
    this.className = 'active';*/
alert(href.childNodes[1].className);
   href.childNodes[1].className = "active";
alert(href.childNodes[1].className);
}
</script>

<ul><li class="leftmenu">
    <a class="passive" href="#">GTL</a>
</li>
<li class="leftmenu">
    <a class="passive" href="#">Сотрудничество</a>
</li>
</ul>
<script>
 var link = document.getElementsByTagName("ul")[0];
       
      link.onclick = function () {
       alert(this.childNodes[1].className);
        this.childNodes[1].className == "passiv" ? this.childNodes[1].className = "active" : this.childNodes[1].className = "passiv";
alert(this.childNodes[1].className);
        
      return false;
      };   
</script>