window.onload = EnableLinksToNewWindows;

function EnableLinksToNewWindows()
{
  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";
    }
  }
}