An old javascript I used for setting active menu classes (with @ppk his W3CDOM checker)

activemenu.js

  var W3CDOM = (document.getElementsByTagName && document.createElement);

  function init(){
    if(!W3CDOM) return;

    setActiveMenuItem();
  }

  function setActiveMenuItem(){
    menuobject = document.getElementsByTagName('li');
    for(var j=0; j<menuobject.length; j++){
       menuAObject = menuobject[j].getElementsByTagName('a');
       for(var s=0; s<menuAObject.length; s++){
          if(menuAObject[s] == self.location.href)
             menuAObject[s].className+=' active';
       }
    }
  }

  window.onload=init;

This is obviously more for reference sake, than to show off my l33t skills =]

Currently I use this one, with jQuery (by Remy Sharp - http://docs.jquery.com/Tutorials:Auto-Selecting_Navigation)

$(function(){
  /* Find active menu item */
  var path = location.pathname.substring(1);
  if(path) 
    $('#secondary a[href$="' + path + '"]').parent("li").attr('class', 'active');
});