Viktor Androshuck

This lightweight plugin automatically scans the content of your posts and pages and adds <code>rel="nofollow noopener noreferrer"</code> to all external links. 
Internal links are untouched. Improves SEO and security with zero configuration.
<ul>
<li>No admin panel needed</li>
<li>Safe and fast</li>
<li>Does not affect internal links</li>
<li>Helps comply with SEO best practices</li>
</ul>

Auto External Link Nofollow


Does exactly what it says on the tin! No fluff, just "no follow" external links across your site 👍


Great Plugin

Automatically adds rel="nofollow noopener noreferrer" to all external links in post and page content.

windyjonas

Add an info box to posts. Only displayed on single posts pages, You can include:
<ul>
<li>Title</li>
<li>Lead in, free text below title</li>
<li>Bullet list, max number of items is configurable</li>
<li>Copy, free text below list</li>
</ul>
Use the included css, or put your own wp-infobox.css in your theme directory.
Requires php5!

Auto External Link Nofollow vs WP-Infobox