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

Recmnd allows you to show related content to your readers. The recommendations are real-time and change when you update your content or when you publish new articles. All recommendations are served via a widget and you control its style, the information shown for each suggestion, the number of displayed recommendations, the time period of the recommendations, and the depth of the recommendation analysis. In order to use the Recmnd service you must register for an account: <a href="http://www.recmnd.com/plans/" rel="nofollow ugc">http://www.recmnd.com/plans/</a>. This plugin works with our trial accounts as well.
For more information, please visit the Recmnd website at <a href="http://www.recmnd.com/" rel="nofollow ugc">http://www.recmnd.com/</a>.

Recmnd

The plugin allows you to use the Recmnd service (www.recmnd.com - automatic recommendations) with your WordPress blog without coding.

Easy Related Posts vs Recmnd