function externalLinks() { 
  if (!document.getElementsByTagName) return;
  var anchors = document.getElementsByTagName("a");
  for (var i=0; i<anchors.length; i++) {
    var anchor = anchors[i];
    if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external") anchor.target = "_blank";
  }
}
window.onload = externalLinks();

// or jquery
$(function(){
    $('a.external').click(function(){
        window.open(this.href);
        return false;
    });
});