microkid

This powerful plugin makes it super easy to manually select related posts for your content. You can search and select posts that fit your criteria, all from within the write/edit post screen.
Creating a system of related items on your website will result in more page views per unique visitor, and will improve Google’s understanding of your content.
As mentioned by <a href="http://yoast.com/articles/wordpress-seo/#relatedposts" title="WordPress SEO and Related Posts " rel="nofollow ugc">WordPress SEO</a> master Yoast:
<blockquote>
A very good alternative is Microkid’s related post plugin, which lets you manually pick related posts. This might cost a bit more time before you hit publish but might very well be worth your while.
</blockquote>
You can display related posts automatically underneath your content, use a shortcode or use the widget. Or, if you have some development skills, you can use one of the available API functions for full control of the way related posts are displayed on your website.
With its full support for Custom Post Types, this plugin will work perfectly for both basic and advanced WordPress installations.
See the official plugin homepage for <a href="http://www.microkid.net/wordpress/related-posts/" title="Wordpress Related Posts" rel="nofollow ugc">Related Posts for WordPress</a> for more info and discussion.
<h4>Features</h4>
<ul>
<li>Easily find posts or pages that might be related to the one you are writing with an integrated AJAX powered search utility</li>
<li>Easily add and remove relations with a single click</li>
<li>Extreme flexibility in the customization of the way the related posts are displayed on your website</li>
<li>The relationship created works reciprocal, which means that if post A is added as related to post B, post B is automatically added as related to post A as well (this option can be turned off easily through the settings panel)</li>
<li>Custom post types support</li>
<li>Multiple widget support with extended customization options</li>
<li>Shortcode support: add the [related-posts] shortcode to your post content to display related posts anywhere you want</li>
<li>Automatically display a list of related posts underneath your posts content. No need to add any PHP tags in your template files, but several <a href="http://www.microkid.net/wordpress/related-posts/#API" title="Related Posts for WordPress API functions" rel="nofollow ugc">functions</a> available for advanced users.</li>
<li>Available in a growing number of languages</li>
</ul>

So far good

Increase your pageviews and SEO by manually selecting related posts for your content. Easy reciprocal linking of related posts and pages.

cfischer83

Does your website utilize categories and tags? Does it use custom taxonomies? If so, this plugin will find similar content 
based on all your taxonomies. There are two ways to show related posts within your page.
<ul>
<li>The first way to show related content on your post is to use the widget provided. This only works when is_single() is true</li>
<li>The second way to show similar content on your site is to use the pk_related_return($post->ID); function which can be 
called programmatically anywhere you wish!</li>
</ul>
The Widget included with this plugin gives you the option to limit the amount of posts; it allows you to determine which 
fields to show: Featured Image, Author, Date, and Excerpt (Title is required); it allows you to determine which 
variant of the featured image to show: thumbnail, medium, large, or full. As of version 2.0, you can now decide whether the 
‘similar posts’ sorting prefers posts that are newer or posts that were created closer to the date of the post you’re 
viewing.
If you find the Widget doesn’t meet your needs or is too limiting, you can call the functionality programmatically using 
this function:
<pre><code>pk_related_return($post->ID, $args);
</code></pre>
Where $post->ID is the ID of the post for which you are wanting to show related articles.
The $args parameter is an array with the following values available to you (more coming soon):
<ul>
<li>posts_per_page (int defaults to 5)</li>
<li>thumbnail_size (string consisting of one of these values: “thumbnail”, “medium”, “large”, “full”. Defaults to thumbnail).</li>
<li>sort_prefer (string consisting of one of these values: “newest”, “closest”. Defaults to newest).</li>
</ul>
An example might be:
<pre><code><?php
$args = array (
 'posts_per_page' => 6,
 'thumbnail_size' => 'medium',
 'sort_prefer' => 'closest'
);
</code></pre>
The return value of pk_related_return is an array of objects that includes most of the fields within WordPress’s posts 
table plus permalink and featured image.
Future Additions:
Allow the user to specify only certain content types (posts, pages, custom) in a request. This would allow you to specify 
only products get returned, or only blog posts. This would only be an issue if content types share taxonomies.

Related Posts for WordPress vs Similar Posts Ontology