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"

iDo8p

NOTE : This plugin is translation ready
This plugin will automatically create links from words.
If you add word on master blog, all network wil lhave the replacement else it’s just for one blog.
Based on the exelent plugin automatic seo links from Emilio (http://emilio.aesinformatica.com)
As Emilio said :
Forget to put manually your links, just choose a word and a URL and this plugin will replace all matches in the posts of your blog.
For each link, you can set:
<ul>
<li>Title</li>
<li>Target</li>
<li>Rel</li>
<li>CSS</li>
</ul>

WPMU Automatic Links

This plugin will automatically create links from words.

rel=”external” in a New Window vs WPMU Automatic Links