Peadig

rel=”external” is now the XHTML valid version that informs search engine spiders that the link is external. However, using this does not open the link in a new window. target=”_blank” and target=”_new” does this, but is not XHTML valid.
This very simple plugin adds a small JS file to your header to let this happen!
<ul>
<li><a href="http://pleer.co.uk/wordpress/plugins/rel-external/" rel="nofollow ugc">rel=”external”</a> plugin homepage.</li>
<li>More <a href="http://pleer.co.uk/wordpress/plugins/" rel="nofollow ugc">WordPress Plugins</a>.</li>
<li><a href="http://pleer.co.uk/wordpress/plugins/facebook-comments/" rel="nofollow ugc">Facebook Comments</a> Plugin</li>
<li><a href="http://pleer.co.uk/wordpress/plugins/wp-twitter-feed/" rel="nofollow ugc">Twitter Feed</a> Plugin</li>
<li><a href="http://twitter.com/alexmossSEO" rel="nofollow ugc">Follow @alexmossSEO on Twitter</a>.</li>
<li><a href="http://alex-moss.co.uk/" rel="nofollow ugc">Alex Moss</a>.</li>
<li><a href="http://pleer.co.uk/" rel="nofollow ugc">Pleer SEO</a>.</li>
</ul>

rel=”external” in a New Window

Opens a new window with rel="external" instead of having to use target="_blank" or target="_new"

Stefano

When a post is saved, the plugin adds the rel=”nofollow” attributes to post external links. The plugin also provides an apt checkbox to exclude a post from plugin’s action.
Links which already have a rel attribute are just ignored, so you can set some dofollows are well.
By default, the plugin will only act on posts, no pages or other custom post types. To include (some of) them, use a code like the following, for ex. by putting it in your theme’s functions.php:
<pre><code>add_filter( 'rnf_post_types', function( $post_types ) {
 return array( 'post', 'page' ); #specify all desired CPTs, comma-separated
});
</code></pre>

Rel Nofollow

Adds rel="nofollow" to posts external links unless specified otherwise.

rel=”external” in a New Window vs Rel Nofollow