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.

xdark

Easy Related Posts is a WordPress plugin that displays posts related to the one a user is currently reading. 
Easy Related Posts can display related posts in main content area or in sidebars through a widget. 
Equipped with 3 eye catching fully responsive themes, supports basic modifications on them from admin panel. It also has a theme system that let you modify or write your own themes to match you personal needs. 
Through an intuitive algorithm it creates “relations” between posts and rates them based on posts categories and/or tags in a way so these ratings are as much as accurate they can be. 
Last but not least in any way, Easy Related Posts uses the best coding standards and practices with mind in security.
If you like this plugin please support the developers. You can do so using the links inside plugin settings page or by purchasing the PRO version.
PRO version can be purchased from <a href="http://erp.xdark.eu/downloads/easy-related-posts-pro/" rel="nofollow ugc">here</a>. 
To see a full list of features and compare them to the PRO version please visit <a href="http://erp.xdark.eu/version-comparison/" rel="nofollow ugc">this page</a>.
Easy Related Posts wont spam your visitors with content from other sites, ads or in any other way.
<a href="http://erp.xdark.eu/demo/" rel="nofollow ugc">See the demo</a>
This plugin requires PHP version 5.3 and above.
Features:
<ul>
<li>Easy to setup</li>
<li>Choose displayed content: excerpt length, thumbnail resizing and many more.</li>
<li>Choose if related posts should be fetched by categories or by tags</li>
<li>Exclude posts that have certain categories and/or tags from related posts</li>
<li>Prevent related posts from spawning in certain categories or tags</li>
<li>Display related posts as an eye-catching pop up block</li>
<li>Display related posts through a widget or/and in main content area</li>
<li>Options optimized before activation for most of the cases</li>
<li>Really fast, not a single database entry (I really love clean DB’s)</li>
<li>Responsive design</li>
</ul>
Please report any bugs or features you’d like to see in feature releases <a href="http://erp.xdark.eu/bugs/set_project.php?project_id=3" rel="nofollow ugc">here</a>

Easy Related Posts

Displays related posts in the end of each post or/and in sidebar through a widget

Similarity vs Easy Related Posts