James Kemp

This plugin allows you to list any number of related posts from the same category as the current post. You can also randomise these results. It is very lightweight, at only 3KB.
There are a number of options available:
<ul>
<li>Choose the number of Posts to show</li>
<li>Choose whether it displays these posts randomly, or in order specified by you.</li>
<li>Choose whether to display a title, what tags to use, and what text should be displayed (e.g. H3)</li>
<li>Choose a class for the main UL</li>
</ul>
To display the random related posts in its simplest form, simply place the following code in your single.php file – <code><?php relatedPosts(); ?></code>.
To edit the options availble, use the following codes:
<pre><code><?php relatedPosts(
 $ppp = 4, 
 $rand = true, 
 $rPinctitle = true, 
 $rPtitletag = 'h3', 
 $rPtitle = 'Related Posts', 
 $rPulclass = 'related_posts',
 $orderby = 'title', 
 $order = 'ASC'
); ?>
</code></pre>
Where:
<ul>
<li>$ppp = Number of posts to display (Any integer allowed)</li>
<li>$rand = sets the display to random (true/false)</li>
<li>$rPinctitle = Displays the title (true/false)</li>
<li>$rPtitletag = The type of tag to wrap the title in (Default h3, but it can be any valid HTML tag)</li>
<li>$rPtitle = The title to be displayed (This can be any text value)</li>
<li>$rPulclass = This sets the class of the ul (This can be any text value)</li>
<li>$rPexcerpt = Displays the post excerpt (true/false)</li>
<li>$orderby = This allows you to order the posts by certain parameters (title/author/date/modified/menu_order/parent/ID/none/comment_count) If you want to choose the order of your posts, be sure to set $rand to false</li>
<li>$order = This can either be Ascending or Descending (ASC/DESC)</li>
</ul>

Random Related Posts Based on Category

This plugin allows you to list any number of related posts from the same category as the current post. You can also randomise these results.

Marcel Pol

Remco

Benoit Chantre

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.

Random Related Posts Based on Category vs Manual Related Posts