Attaches a external class to all links outside the domain. I got the code from: http://codegrad.hub.ph/marking-external-links-icon-wordpress/ and I extended it through line 14. The only thing you have to do: search a external link icon. You can find a good one on genericon.com.

style.sass

/* external links
a.external
  background: url('../images/external_link.png') no-repeat right 0px

twwc-bootstrap-external-links.php

<?php

/**
 * Displays external link icon for WordPress
 *
 * Code from: http://codegrad.hub.ph/marking-external-links-icon-wordpress/
 *
 * @package TWWC Bootstrap
 * @author Claudio Schwarz
 * @since  twwc-bootstrap 0.1
 */

function twwc_bootstrap_external_link($text) {
  $wpUrl = get_bloginfo('url');
  $return = str_replace('href=', 'class="external" href=', $text);
  $return = str_replace('class="external" href="' . $wpUrl . '', 'href="' . $wpUrl . '', $return);
  $return = str_replace('class="external" href="#', 'href="#', $return);
  return $return;
}
add_filter('the_content', 'twwc_bootstrap_external_link');