David Miller

Similarity displays a list of posts similar to the current post with similarity being determined based on tags, categories, or both. The display of the list is fully customizable including the ability to display the strength of the similarity as a value, percentage, color, or custom text/code. Another option allows for a less related post to be randomly selected and appended to the list. The list can be limited by one or both of a numerical limit or (as of version 2.1) match strength.
This plugin is translation ready and comes with English (U.S.), French, Polish, German, italian, and Spanish translations. Thanks to Li-An (http://li-an.fr/blog/) for providing the French translation. Thanks to Krzysztof Kudlacik (http://born66.net/) for providing the Polish translation. Thanks to Bjoern Buerstinghaus (http://www.buerstinghaus.net/) for providing the German, Italian and Spanish translations.
Prior to Version 1.6 private posts would potentially show in the list of related posts. In 1.6 these were skipped. As of version 1.7 private posts will show for the post author or others with the ability to read private posts. It is strongly recommended that users who have private posts install version 1.6 or higher.
The plugin allows for three function calls anywhere in your page templates (all use the same options):
<ul>
<li><code><?php sim_by_tag(); ?></code> – determines similarity based on the tags applied to the posts</li>
<li><code><?php sim_by_cat(); ?></code> – determines similarity based on the categories assigned to the posts</li>
<li><code><?php sim_by_mix(); ?></code> – determines similarity based on the tags and categories assigned to the posts</li>
</ul>
There are also three function calls that are better suited for use in sidebars (specifically when the sidebar is used on the main page)
<ul>
<li><code><?php sim_by_tag_multi(); ?></code> – determines similarity based on the tags applied to the first post on milti-post pages</li>
<li><code><?php sim_by_cat_multi(); ?></code> – determines similarity based on the categories assigned to the first post on milti-post pages</li>
<li><code><?php sim_by_mix(_multi); ?></code> – determines similarity based on the categories and tags assigned to the first post on milti-post pages weighting each according to the ratio you assign</li>
<li>To display a Similarity list as a widget use a text widget with one of the following shortcodes – <code>[SIM-BY-TAG]</code> <code>[SIM-BY-CAT]</code> <code>[SIM-BY-MIX]</code> for single post pages only or for all pages use <code>[SIM-BY-TAG-MULTI]</code> <code>[SIM-BY-CAT-MULTI]</code> or <code>[SIM-BY-MIX-MULTI]</code></li>
<li>To display a Similarity list without altering your templates simply select the function that you would like to auto-display at the bottom of the options page.</li>
</ul>

Similarity

Similarity displays a list of posts similar to the current post with similarity being determined based on tags, categories, or both.

WebshopLogic

WP Related Posts and Products plugin offers different types of similar contents for your users. For example, some related products can be displayed to your similar posts. This makes connections of your site contents visible to your clients, increasing business efficiency.
Would you like to offer some related products to your blog posts? Do you need to offer up-sell or cross-sell products? Do you have an event calendar plugin, and want to suggest some programs connected to an article? Do you have a custom movie catalog plugin and want to associate some articles to your movies?
Using cross type relationship management functions, WRI makes it possible to associate a post, page, or other custom post type to other posts, pages or custom post types, ensures widely configurable relatedness settings, sophisticated cross-relation adjustments. WRI uses the most popular YARPP relationship handling plugin in the background, extending its functionality, retaining its advantages.
WRI combines automatic, manual and common categorization based relationship management. Some functions are available in the PRO version.
You can set attributes that define how to display different type of related items for every reference item type. Such a matrix-like way you can specify all necessary variations of display settings.
This version ensures built in WooCommerce custom product support.
<h4>More information, documentation</h4>
LITE: <a href="https://webshoplogic.com/products/wordpress_plugins/wp-related-posts-and-products-lite-plugin/#tab-faq&noscroll" rel="nofollow ugc">WP Related Posts and Products Lite Documentation</a> 
PRO: <a href="https://webshoplogic.com/products/wordpress_plugins/wp-related-posts-and-products-pro-plugin/#tab-faq&noscroll" rel="nofollow ugc">WP Related Posts and Products Pro Documentation</a>
<h4>Available languages</h4>
<ul>
<li>English</li>
<li>Spanish – translated by Andrew Kurtis – WebHostingHub </li>
<li>Hungarian – translated by WebshopLogic</li>
</ul>

WP Related Posts and Products (WRI)

WP Related Items plugin offers different types of similar contents for your users. For example, some related products can be displayed to your posts.

Similarity vs WP Related Posts and Products (WRI)