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.

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 Nofollow vs WPMU Automatic Links