marcinmrowiec

To display related posts just add shortcode [getrelatedposts] or with attributes [getrelatedposts title=”your title” relation=”tags/category” numberofposts=”somenumber” ] to your post page. 
You can change global plugin settings in RSRPP Settings.
At first use plugin will create template “rsrpp-template.php” in active theme directory.

Really Simple Related Posts Plugin

Really Simple Related Posts Plugin - plugin for displaying related posts using shorcodes with your own template.

Marcin Pietrzak

Display cool, animated fly-out or fade box with related content.
When a reader scrolls to the bottom of a single post, page, or custom post type, an animate box will be shown allowing the reader to select the previous or random available post or posts in the selected configuration:
<ol>
<li>Just previous</li>
<li>Previous in category</li>
<li>Previous in tag</li>
<li>Random</li>
<li>Related using YARPP (only post/pages)</li>
</ol>

upPrev

Great plugin. Works great, lightweight and doesn't take a lot of resources to manage. 

Great Plugin

Display cool, animated fly-out or fade box with related content.

Really Simple Related Posts Plugin vs upPrev