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"

WebFactory

Opens external links in a new tab or a or new window. You can set URLs that should either be forced to open in a new window or ignored.
The plugin produces XHTML Strict compliant code and is search engine optimized (SEO). 
This is done using JavaScript’s <code>window.open()</code>-function. It adds only a few lines of vanilla JavaScript to the page, and does not require any external libraries like jQuery.
Most other plugins perform a hack by altering the <code>target</code> parameter (i.e. <code><a href="http://somewhere.example" target="_blank"></code>). That method is not XHTML Strict compliant. 
This plugin handles the links client-side, which lets search engines follow the links properly. Also, if a browser does not support JavaScript, the plugin is simply inactive, and does not result in any errors.
If you need a more advanced plugin, with more options try our <a href="https://wordpress.org/plugins/wp-external-links/" rel="ugc">free WP External Links</a> plugin.
Credits 
Inspired by the <a href="http://www.zappelfillip.de/2005-12-05/zap_newwindow/" rel="nofollow ugc">Zap_NewWindow</a> plugin by <a href="http://www.zappelfillip.de/" title="His website is mostly in German" rel="nofollow ugc">Tom Köhler</a>. 
The banner is a <a href="http://www.flickr.com/photos/monja/1367946568/in/photostream/" rel="nofollow ugc">photo</a> by <a href="http://www.monja.it/" rel="nofollow ugc">Monja Da Riva</a>.
Translations 
Danish by <a href="https://kristianrisagerlarsen.dk" rel="nofollow ugc">Kristian Risager Larsen</a> 
Dutch by <a href="http://www.collectief-it.nl/" rel="nofollow ugc">Paul Staring</a> 
Lithuanian by <a href="http://Host1Free.com" rel="nofollow ugc">Vincent G</a> 
Other translations will be appreciated!
Known bugs 
The plugin conflicts with other plugins that change the links’ `onClick´ attribute.
Original developer 
Kristian Risager Larsen – <a href="http://kezze.dk" rel="nofollow ugc">kezze.dk</a>

External Links in New Window / New Tab

Open external links in a new window or new tab. SEO optimized and XHTML Strict compliant.

rel=”external” in a New Window vs External Links in New Window / New Tab