// change your domain name:
var yourURL = "sample.com";
function outLinks() {
  var outLink;
  if (document.getElementsByTagName('a')) {
    for (var i = 0; (outLink = document.getElementsByTagName('a')[i]); i++) {
      if (outLink.href.indexOf(yourURL) == -1) {
        outLink.setAttribute('target', '_blank');
      }
    }
  }
}
window.onload = function() {
    outLinks();
}

http://www.maratz.com/blog/archives/2004/11/29/setting-target_blank-to-external-links/

If you want your pages to be valid XHTML-Strict, but you also wish to open external links in new window, the following snippet is doing just that—sets attribute target=“_blank” to all external links in a document to keep your code valid XHTML-Strict.  You’ll just have to change variable yourURL. Don’t enter http://www. part of your domain, otherwise something like http://sample.com/ will be treated like any other external link.

Setting target=“_blank” to external links and standards

javascript, html

JavaScript