Arunas Liuiza

A tiny and simple hackable plugin to manually assign and display related posts. Incudes a widget, a shortcode (<code>[tinyrelated]</code>), a couple of template tags (<code>the_related_posts()/get_related_posts()</code>) for easy integration.
You can pick up to 10 related posts via a meta box in <code>Edit Post</code> window. If no posts were selected, there is an option to display random posts instead. Currently, there is no automatic generation of related posts.
The plugin is translation ready and has Lithuanian translation.
Also try out my other plugins:
<ul>
<li><a href="http://tiny.lt/gust" rel="nofollow ugc">Gust</a> – a Ghost-like admin panel for WordPress, featuring Markdown based split-view editor.</li>
<li><a href="http://tiny.lt/tinycoffee" rel="nofollow ugc">tinyCoffee</a> – a PayPal donations button with a twist. Ask people to treat you to a coffee/beer/etc. </li>
<li><a href="http://tiny.lt/tinytoc" rel="nofollow ugc">tinyTOC</a> – automatic Table of Contents, based on H1-H6 headings in post content.</li>
<li><a href="http://tiny.lt/tinyip" rel="nofollow ugc">tinyIP</a> – Premium – stop WordPress users from sharing login information, force users to be logged in only from one device at a time.</li>
</ul>
An enormous amount of coffee was consumed while developing these plugins, so if you like what you get, please consider treating me to a <a href="http://kava.tribuna.lt/en/" rel="nofollow ugc">cup</a>. Or two. Or ten.

tinyRelated

A tiny and simple plugin to manually assign and display related posts.

AMIMOTO

horike

wokamoto

Kazuto Takeshita

webnist

gatespace

*Related posts based on categories, tags. 
*Can be replaced and sort of related posts at admin screen. 
*I can be added as an add-on another related posts algorithm.
You can make addon to use <a href="https://github.com/horike37/wp-simple-related-posts-addon-template/" rel="nofollow ugc">Addon Template</a>.
You can use JSON REST API Endpoint. Require plugin <a href="https://wordpress.org/plugins/json-rest-api/" rel="ugc">JSON REST API</a> 
 http://example.com/wp-json/posts/%post_id%/sirp_related/ 
 http://example.com/wp-json/sirp_related/%post_id%/ 
 http://example.com/wp-json/sirp_related/%post_id%/?filter[num]=5
<h4>Translators</h4>
<ul>
<li>Japanese(ja) – <a href="https://profiles.wordpress.org/horike" rel="nofollow ugc">Horike Takahiro</a></li>
<li>Thai(th_TH) – <a href="http://www.xn--12cg1cxchd0a2gzc1c5d5a.com" rel="nofollow ugc">TG Knowledge</a></li>
</ul>
<h4>Contributors</h4>
<ul>
<li>@<a href="https://profiles.wordpress.org/shinichin" rel="nofollow ugc">ShinichiN</a></li>
<li>@<a href="https://profiles.wordpress.org/webnist" rel="nofollow ugc">Webnist</a></li>
<li>@<a href="https://profiles.wordpress.org/mt8biz" rel="nofollow ugc">mt8biz</a></li>
</ul>

WP Simple Related Posts

Related Posts plugin. It's flexible and fast and simple.

tinyRelated vs WP Simple Related Posts