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;