alphasis

The widget area lists posts of the same category as the current post. 
This plugin requires the WP_Widget class introduced in WordPress Version 2.8. 
This widget works when any single Post page is being displayed.
<h4>Official Page</h4>
<ul>
<li>Plugin’s Official Page : <a href="http://alphasis.info/developments/wordpress-plugins/widgets-of-posts-by-same-categories/" title="English" rel="nofollow ugc">English</a> / <a href="http://alphasis.info/2010/11/widgets-of-posts-by-same-categories/" title="Japanese" rel="nofollow ugc">Japanese</a></li>
<li>Author’s Website : <a href="http://alphasis.info/" title="alphasis.info" rel="nofollow ugc">alphasis.info</a></li>
</ul>
<h4>Options</h4>
<ul>
<li>Number of Posts to Show by Categories</li>
<li>Order By
<ul>
<li>Sort by title</li>
<li>Sort by creation date</li>
<li>Sort by last modified date</li>
<li>Sort by numeric post ID</li>
<li>Random</li>
</ul>
</li>
<li>Order
<ul>
<li>Ascending</li>
<li>Descending</li>
</ul>
</li>
<li>Creates a link to each category?
<ul>
<li>Yes</li>
<li>No</li>
</ul>
</li>
<li>Exclude (Categories)
<ul>
<li>The IDs of any categories you want to exclude, separated by commas.</li>
</ul>
</li>
<li>Separator
<ul>
<li>What to separate each category by.</li>
</ul>
</li>
</ul>
<h4>Localization</h4>
<ul>
<li>Japanese (ja) – <a href="http://alphasis.info/" title="alphasis.info" rel="nofollow ugc">alphasis</a> (plugin author)</li>
<li>Current release supports English and Japanese only.</li>
</ul>

Widgets of Posts by Same Categories

The widget area lists posts of the same category as the current post.

keesiemeijer

Quickly increase your readers’ engagement by adding related posts in the sidebar or after post content with a widget or shortcode.
Posts with the most terms in common will display at the top!
This plugin is capable of finding related posts in multiple taxonomies and post types. Include or exclude terms from the search for related posts. Change the look and feel by using your own templates in a (child) theme.
<a href="http://keesiemeijer.wordpress.com/related-posts-by-taxonomy/" rel="nofollow ugc">plugin documentation</a>
Plugin features:
<ul>
<li>Widget and Shortcode.</li>
<li>Display related posts as post thumbnails, links, excerpts or full posts.</li>
<li>Small Footprint. Doesn’t slow down your site!</li>
<li>Automatic display of related posts after the post content.</li>
<li>Exclude or include terms and posts.</li>
<li>Search for related posts in single or multiple taxonomies and post types.</li>
<li>Limit the search for related posts by date, number or post meta.</li>
<li>Use your own HTML templates for display of the related posts.</li>
<li>Extensive <a href="http://keesiemeijer.wordpress.com/related-posts-by-taxonomy/" rel="nofollow ugc">plugin documentation</a>.</li>
<li>Follows WordPress coding standards and plugin best practices.</li>
<li>Highly Adjustable!</li>
</ul>
For the following features you need to be somewhat familiar with WordPress <a href="https://developer.wordpress.org/plugins/hooks/" rel="nofollow ugc">hooks</a>. The <a href="http://keesiemeijer.wordpress.com/related-posts-by-taxonomy/" rel="nofollow ugc">plugin documentation</a> has many examples to help you along.
Extended features:
<ul>
<li>Use a persistent cache for the related posts query. (opt-in feature)</li>
<li>Use the WordPress REST API to get related posts. (opt-in feature)</li>
<li>Load related posts after the whole page has loaded (with Ajax). (opt-in feature)</li>
<li>Use plugin functions in your theme templates to display related posts yourself.</li>
<li>Use Filters to change the default behavior of the plugin. </li>
</ul>
Follow this plugin on <a href="https://github.com/keesiemeijer/related-posts-by-taxonomy" rel="nofollow ugc">GitHub</a>. 
Search the <a href="https://keesiemeijer.github.io/related-posts-by-taxonomy" rel="nofollow ugc">code reference</a>
See the <a href="https://wordpress.org/extend/plugins/related-posts-by-taxonomy/faq/" rel="ugc">FAQ</a> to have related posts automatically display after the post content without using the shortcode or widget.
Default usage for the shortcode is:
<pre><code>[related_posts_by_tax]
</code></pre>
Attributes for the shortcode are:
<ul>
<li><code>post_id</code></li>
<li><code>taxonomies</code></li>
<li><code>post_types</code></li>
<li><code>posts_per_page</code></li>
<li><code>order</code></li>
<li><code>orderby</code></li>
<li><code>before_shortcode</code></li>
<li><code>after_shortcode</code></li>
<li><code>title</code></li>
<li><code>show_date</code></li>
<li><code>before_title</code></li>
<li><code>after_title</code></li>
<li><code>include_terms</code></li>
<li><code>include_parents</code></li>
<li><code>include_children</code></li>
<li><code>exclude_terms</code></li>
<li><code>exclude_posts</code></li>
<li><code>format</code></li>
<li><code>gallery_format</code></li>
<li><code>image_size</code></li>
<li><code>columns</code></li>
<li><code>caption</code></li>
<li><code>link_caption</code></li>
<li><code>limit_posts</code></li>
<li><code>limit_month</code></li>
<li><code>meta_key</code></li>
<li><code>meta_value</code></li>
<li><code>meta_compare</code></li>
<li><code>meta_type</code></li>
<li><code>public_only</code></li>
<li><code>include_self</code></li>
<li><code>post_class</code></li>
</ul>
Example to show 10 related posts instead of the default 5.
<pre><code>[related_posts_by_tax posts_per_page="10"]
</code></pre>
See the <a href="https://keesiemeijer.wordpress.com/related-posts-by-taxonomy/#shortcode" rel="nofollow ugc">documentation</a> for more information about these attributes.
<h4>Translations</h4>
<ul>
<li>Dutch </li>
<li>French (by <a href="http://www.artisanathai.fr" rel="nofollow ugc">Annie Stasse</a>) </li>
<li>Spanish (by <a href="http://www.ludobooks.com" rel="nofollow ugc">Ludobooks – Cuentos personalizados</a>) </li>
<li>Catalan (by <a href="http://www.ludobooks.com" rel="nofollow ugc">Ludobooks – Cuentos personalizados</a>) </li>
<li>Polish (by <a href="http://rekolekcje.net.pl" rel="nofollow ugc">koda0601</a>)</li>
</ul>

My goodness!

Display a list of related posts on your site based on the most terms in common. Supports thumbnails, shortcodes, a widget and more.

Widgets of Posts by Same Categories vs Related Posts by Taxonomy