外部リンク自動検出のやつ source:rskull

gistfile1.js

javascript:function AutoCheckURL() {
  var HostURL = new RegExp(location.hostname + "(\\:[0-9]+)?"),
  CheckURL = document.links;

  for(var i = 0; i < CheckURL.length; ++i)
    if(!HostURL.test(CheckURL[i].href.split('/')[2]))
      CheckURL[i].target = '_blank';
}
AutoCheckURL();