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.onclick    = function(event){return popupWindow(this, event);}
            anchor.onkeypress = function(event){return popupWindow(this, event);}
        }
    }
}

function popupWindow(anchor, event){
    var keyCode;
    if (event && event.type == 'keypress') {
        if (event.keyCode)
            keyCode = event.keyCode;
        else if (event.which)
            keyCode = event.which;
        // 13 == Return key. 32 == space key
        if (keyCode != 13 && keyCode != 32)
            return true;
    }
    return !window.open(anchor);
}

Event.observe(window,'load', externalLinks, false);