Erin McIntyre

This plugin searches links on your site for the domain that you specify in the settings, adds your affiliate ID, and makes those links open in a new tab when clicked.
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.
Credits 
Inspired by the <a href="https://wordpress.org/plugins/open-external-links-in-a-new-window/" rel="ugc">External Links in New Window / New Tab</a> plugin.
Known bugs 
The plugin conflicts with other plugins that change the links’ `onClick´ attribute.

Affiliatize Me

Codious Technologies

Codious External Links Opener is a lightweight WordPress plugin that automatically opens all external links in a new browser tab. 
It also adds <code>rel="noopener noreferrer"</code> to external links for improved security and performance.
No setup required. Simply install and activate the plugin.
Features:
<ul>
<li>Automatically adds <code>target="_blank"</code> to external links</li>
<li>Adds <code>rel="noopener noreferrer"</code> for security and performance</li>
<li>No settings or configuration required</li>
<li>Lightweight and fast</li>
<li>Works with posts, pages, and custom post types</li>
</ul>
<h3>Credits</h3>
Developed by Codious Technologies

Codious External Links Opener

Automatically adds target="_blank" and rel="noopener noreferrer" to external links in post content to improve user experience and security.

Affiliatize Me vs Codious External Links Opener