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;

http://www.sitepoint.com/standards-compliant-world/

Usage:
<a href="http://www.whaterver-your-site-is-called.com/" rel="external">Some other site</a>

Javascript open links in a new window, without _blank

links

JavaScript