function openNewWindow() {
    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 = openNewWindow;