function externalLinks() {
  if (! document.getElementsByTagName ) return;
  var anchors = document.getElementsByTagName("a");
  var isIE = (document.documentElement.getAttribute("style") == document.documentElement.style);
  for (var i=0; i<anchors.length; i++) {
    if(anchors[i].getAttribute("href") && anchors[i].getAttribute("rel") == "external"){
      if(isIE) anchors[i].setAttribute("onclick",new Function("window.open(this.href); return false;"));
      else anchors[i].setAttribute("onclick","window.open(this.href); return false;");
    }
  }
}

window.onload = externalLinks;