dcannon1

Small, lightweight (under 7k) widget that shows posts from the same category as the current post. Several options for customization along with numerous ordering criteria for posts. Displayed only on single post pages. Based on Related Links by Category by Andrew Stephens
Options
<ul>
<li>Title – Set the title of the widget (or leave blank)</li>
<li>Number of Posts to Show – Set how many posts are listed in the widget</li>
<li>Order By – Choose how the related posts will be ordered (including random posts)</li>
<li>Show Category Names – Toggle whether category names are displayed above groups of posts</li>
<li>Link Cat Name to Archive – If category names are being shown, choose to have them link to the category archive page for that category</li>
<li>List Posts Multiple Times? – If a post is classified under multiple categories use this toggle to determine if it will be shown under each category, or only under the first category it shares with the current post. This should be set to no if you are not displaying the category names</li>
</ul>

PickPlugins

Related Posts By PickPlugins plugin is one of the most useful plugin to display related post under post content on single post or page or custom post types, its also support to display related post under excerpt on archive pages. you can conditionally choose to display related content under excerpt or content by archive pages like tag, category, date, author, search page and custom taxonomy pages.
<h3>Related Post by http://pickplugins.com</h3>
<ul>
<li><a href="https://www.pickplugins.com/documentation/related-post/?ref=wordpress.org" rel="nofollow ugc">Documentation! »</a></li>
<li><a href="https://demo.pickplugins.com/related-post/?ref=wordpress.org" rel="nofollow ugc">Live Demo! »</a></li>
<li><a href="https://www.pickplugins.com/item/related-post-for-wordpress/?ref=wordpress.org" rel="nofollow ugc">Buy Pro! »</a></li>
</ul>
Tutorials
<ul>
<li><a href="https://www.youtube.com/watch?v=9SZKa0QYgsc" rel="nofollow ugc">How to install & setup</a></li>
</ul>
Related post under every paragraph
You can choose paragraph position like first, second, third and before last paragraph to display related post.
Before & After content and excerpt
Related post plugin allows you to display related content link before and after the content, this feature also available for excerpt under archive pages.
Related post under archive page
You can display related post under various archive pages like home page, front page, blog page, date, search, author, year, date, month and etc.
Related post by post types
You can choose custom post types to display Related post under single page.
Manual Post selection 
You can choose manually picked the post to display as related post for each post.
Advance query 
You can set max number post to display and choose query order and orderby parameters, related posts query based on current post category, tags and custom taxonomies.
Ready layout
There is 3 different type layout currently available to display related post, you can choose grid, slider and list style layout. you can set custom width for items and margin, padding, text align.
Sortable post elements
You can sort post elements like post title, post thumbnail and excerpt as you want.
Hide any elements
You can hide or display post elements like post title, thumbnail or excerpt.
Customize elements style
You can set custom font size, font color, padding, margin for post title, post excerpt and set custom height for post thumbnail, select custom thumbnail size and etc. you can also write custom CSS for each elements.
Track click
You can enable tracking click on related post, this will help you understand which post getting more attention from related posts. you will see top 10 post from stats page.
<h3>Premium Features</h3>
Popup related post 
You can display related post on popups under single blog post or custom post types,
Popup positions 
you can set 8 ready position for popups
Popup custom delay 
You can set custom delay to display popup, so popup will be hidden until certain amount of time and then it will display.
Popup display on scroll 
You can choose to display popup based on scroll down, popup will be hidden until certain amount of scroll down.
Popup display scroll down to article 
Popup will display when user scroll down to reached end of the article class or content.
Popup display scroll down to page 
Popup will display when user scroll down to reached end of the page or footer.
Custom HTML after each elements 
You can display custom HTML under each elements like post title, post excerpt and post thumbnails. you can also display 3rd party shrotcode as output after each elements.
Link Target 
You can set custom link target for each elements link like post title, post thumbnail and read more link, you can set _blank, _parent or etc for each link.
Display via shortcode
You can display related post any where via shortcode by using on your theme files
<pre><code><?php echo do_shortcode( '[related_post post_id=""]' ); ?>
</code></pre>

Related Posts by Category Widget vs Related Posts By PickPlugins