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"

rrolfe

The CLM plugin allows wordpress admins to have more control over the links left by users who leave 
comments on their blogs. Admins are provided with the ability to remove all author links from comments 
unless the user has made an admin specified number of comments. You are also provided with the ability 
to override that setting with whitelist and blacklist features. The full list of features provided by 
the CLM plugin is below
Features
<ul>
<li>Enables admins to remove nofollow attribute from comment author links</li>
<li>Allows for optional removal of nofollow attribute from links left in the body of comments</li>
<li>Provides admins with the ability to open comment author and body links in a new browser window.</li>
<li>Provides ability for admins to require a specified number of comments before enabling comment author links</li>
<li>Has built in whitelist and blacklist functions to override general setting of required number of comments</li>
</ul>

Comment Link Manager

CLM enables admins to disable author links, open links in new window, and remove the nofollow tag from links that are left in comments by visitors.

rel=”external” in a New Window vs Comment Link Manager