danielauener

A widget that simply gives you related posts by taxonomy. It provides four 
settings to customize every widget and there are no global settings to worry 
about.
Widget settings
<ol>
<li>The title of the widget (default: Related Posts)</li>
<li>The taxonomy by which terms the posts are related (default: Tags)</li>
<li>How many related posts to show (default 5)</li>
<li>excludet terms</li>
</ol>
The widget is visible only when <code>is_single()</code> is true, it doesn’t shows up on 
archives, search templates etc.
Translation credits
Thanks to Borisa Djuraskovic from <a href="http://www.webhostinghub.com" rel="nofollow ugc">http://www.webhostinghub.com</a> for the Serbian translation!

Simply Related Posts

A widget that simply gives you related posts by taxonomy. Four settings to customize the widget: title, taxonomy, related posts count, excludet terms

Marcel Pol

Benoit Chantre

Remco

A simple ‘related posts’ plugin that lets you select related posts manually. Supports any post types in WordPress, including custom ones.
Features:
<ul>
<li>Add related posts to your blog posts, pages etc.</li>
<li>Choose from posts, pages or custom post types.</li>
<li>Re-order related posts via drag and drop.</li>
<li>Lightweight in code and database-requests.</li>
<li>Includes Widget that shows the related posts.</li>
<li>List of posts can also be added to the content of your post.</li>
<li>Custom markup possible, build your own caroussel or anything you fancy.</li>
<li>Support for multilanguage plugins, like WPML and Qtranslate-X.</li>
<li>Duplicate plugin Related (Doubled Up) is included to build a second list.</li>
</ul>
The plugin was written to have the option to add related posts to each blog post using a simple but functional plugin. You can select the related posts yourself manually.
To display the related posts, there are three options:
<ul>
<li>You can use the widget that is included.</li>
<li>Use the content filter inside the settings.</li>
<li>Add PHP code to your template, see the installation docs.</li>
</ul>
For advanced options, see the installation docs.
<h4>Compatibility</h4>
This plugin is compatible with <a href="https://www.classicpress.net" rel="nofollow ugc">ClassicPress</a>.
<h4>Contributions</h4>
This plugin is also available in <a href="https://codeberg.org/cyclotouriste/related" rel="nofollow ugc">Codeberg</a>.

Manual Related Posts

Does exactly what is says it will. Very easy to integrate where you need it. Kudos to the dev who made it.

Simple and Easy to integrate

Very handy plugin for manually set your related posts !

Simple and Efficient

A simple 'related posts' plugin that lets you select related posts manually.

Simply Related Posts vs Manual Related Posts