function markActiveLink() {

    //Look through all the links in the sidebar
   $("div#menu a").filter(function() {

      //Take the current URL and split it into chunks at each slash
      var currentURL = window.location.toString().split("/");

      //return true if the bit after the last slash is the current page name
      return $(this).attr("href") == currentURL[currentURL.length-1];

    //when the filter function is done, you're left with the links that match.
    }).addClass("selected");

   //Afterwards, look back through the links. If none of them were marked,
   //mark your default one.
   if($("div#menu a").hasClass("active") == false) {
      $("div#menu h2:nth-child(2) a").addClass("active");
    }
 }

markActiveLink();