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.

Daniel Floeter

Mrinal Roy

Same Category Posts shows a list of related posts with a same Post Type to the current post. The widget is only shown on single post pages. Forked from <a href="https://wordpress.org/plugins/related-posts-widget" rel="ugc">Related Posts Widget</a>.
<h4>Tip Top Press</h4>
We’re <a href="http://tiptoppress.com/" rel="nofollow ugc">Tip Top Press</a> and create widgets for WordPress. If you want to know about what we’re working on and you are interested in backgrounds then you can read all newes storys on our <a href="http://tiptoppress.com/blog/?utm_source=wp.org&utm_medium=readme.txt&utm_campaign=same+category+posts" rel="nofollow ugc">blog</a>.
<h4>Features</h4>
<ul>
<li>Shows a list of related posts.</li>
<li>Option which Post Type should be related to the current post.</li>
<li>Custom Post Types support.</li>
<li>Child category and terms support.</li>
<li>Archive page support.</li>
<li>Option exclude one or multi categories or terms.</li>
<li>Option to filter by popular posts (by comment count).</li>
<li>Option <a href="http://tiptoppress.com/new-dynamic-layout-feature-separate-categories/" rel="nofollow ugc">separate categories</a> if more than one is assigned.</li>
<li>Set how many posts to show (overall and by category).</li>
<li>Option exclude current post, sticky posts or children.</li>
<li>Option exclude categories and terms without exclude their children.</li>
<li>Placeholders in title string (e.g. “There are a lot of %cat%-News.” -> “There are a lot of Tech-News.”).</li>
<li>Filter hook for the post titles ‘widget_title’.</li>
<li>Option to show post thumbnail and set width & height.</li>
<li>Option to <a href="http://tiptoppress.com/css-image-crop/" rel="nofollow ugc">crop thumbnails dimensions with CSS</a>.</li>
<li>Option to put thumbnail on top.</li>
<li>Option to make the widget title link to the category page.</li>
<li>Option to show/hide the title.</li>
<li>Option to show the post excerpt and how long (in words).</li>
<li>Option change excerpt ‘more’ text.</li>
<li>Option to show the post date, author and comment count.</li>
<li>Multiple widgets.</li>
</ul>
<h4>Placeholder</h4>
In text boxes %cat% will replaced with the (first assigned) category name, e.g. “There are a lot of %cat%-News.” -> “There are a lot of Tech-News.”
And %cat-all% will replaced with all assigned category name, e.g. “Special offers for %cat-all%!” -> “Special offers for houses, flats, apartments.”
<h4>Contribute</h4>
While using this plugin if you find any bug or any conflict, please submit an issue at 
<a href="https://github.com/DanielFloeter/same-category-posts" rel="nofollow ugc">Github</a> (If possible with a pull request).

Same Category Posts

Show posts related to the current category or other custom post types.

Random Related Posts Based on Category vs Same Category Posts