function toggle_show(id) {
        document.getElementById(id).style.display = document.getElementById(id).style.display == 'none' ? 'block' : 'none';

}

function toggle_show(id) {
        document.getElementById(id).style.display = getStyle(document.getElementById(id),'display') != 'none' ? 'none ':'block' ;
}
//-----
function getStyle(element,CSSProperty)
{ return (typeof getComputedStyle == "undefined" ? element.currentStyle : getComputedStyle(element, null))[CSSProperty];
}

<input type=button value="Клик" onclick="toggle_show('Clk')"><div id="Clk" style="background:green;width:260px;height:170px;" "></div>
<script>
function toggle_show(id) {
        var el=document.getElementById(id)
el.style.display = el.style.display == 'none' ? 'block' : 'none';

}
</script>

<div class="menuleft">
   <ui>
      <li><a href="#">Заголовок</a></li>
      <li>Меню 1</li>
      <li>Меню 2</li>
      <li>Меню 3</li>
   </ui>
   <ui>
      <li><a href="#">Заголовок 2</a></li>
      <li>Меню 4</li>
      <li>Меню 5</li>
      <li>Меню 6</li>
   </ui>
</div>

document.getElementsByClassName('menuleft')[0].onclick = function()
{
   var e = event || window.event;
   if(e.target.tagName == 'A')
   {
       var li = this.parentNode.parentNode.getElementsByTagName('li');
       for(i=1; i<li.length; i++) li[i].style.display = li[i].style.display ? '' : 'none';
   }
}