Why you need it
Related post plugins allow you to present your reader with other content they may find useful or interesting. This helps keep your reader browsing your site and also provides additional internal links, which help search engines discover all your content.

Microkids Related Posts plugin provides a simple, effective solution.

Ease of use
The options are very easy to configure but has advanced capabilities for those that need them. As you can see the settings screen is very straightforward (click image to enlarge):

Features

• You can customize the text that shows up beneath your blog post.
• My favorite feature is that the links are reciprocal. So if I add a related post link from a new blog post to an old one, it will automatically update the old post with a link to the new. Very handy.
• The related posts are manually selected via an AJAX powered search utility on the blog post edit screen. I like having full control over which posts are going to show up, rather than an automated system that tries to predict what is relevant. This also keeps the configuration options simple.

• For advanced users there are some API functions available and you can customize the output with CSS.

Support/documentation
Ample information is provided at :
http://www.microkid.net/wordpress/related-posts/