David Miller

Similarity displays a list of posts similar to the current post with similarity being determined based on tags, categories, or both. The display of the list is fully customizable including the ability to display the strength of the similarity as a value, percentage, color, or custom text/code. Another option allows for a less related post to be randomly selected and appended to the list. The list can be limited by one or both of a numerical limit or (as of version 2.1) match strength.
This plugin is translation ready and comes with English (U.S.), French, Polish, German, italian, and Spanish translations. Thanks to Li-An (http://li-an.fr/blog/) for providing the French translation. Thanks to Krzysztof Kudlacik (http://born66.net/) for providing the Polish translation. Thanks to Bjoern Buerstinghaus (http://www.buerstinghaus.net/) for providing the German, Italian and Spanish translations.
Prior to Version 1.6 private posts would potentially show in the list of related posts. In 1.6 these were skipped. As of version 1.7 private posts will show for the post author or others with the ability to read private posts. It is strongly recommended that users who have private posts install version 1.6 or higher.
The plugin allows for three function calls anywhere in your page templates (all use the same options):
<ul>
<li><code><?php sim_by_tag(); ?></code> – determines similarity based on the tags applied to the posts</li>
<li><code><?php sim_by_cat(); ?></code> – determines similarity based on the categories assigned to the posts</li>
<li><code><?php sim_by_mix(); ?></code> – determines similarity based on the tags and categories assigned to the posts</li>
</ul>
There are also three function calls that are better suited for use in sidebars (specifically when the sidebar is used on the main page)
<ul>
<li><code><?php sim_by_tag_multi(); ?></code> – determines similarity based on the tags applied to the first post on milti-post pages</li>
<li><code><?php sim_by_cat_multi(); ?></code> – determines similarity based on the categories assigned to the first post on milti-post pages</li>
<li><code><?php sim_by_mix(_multi); ?></code> – determines similarity based on the categories and tags assigned to the first post on milti-post pages weighting each according to the ratio you assign</li>
<li>To display a Similarity list as a widget use a text widget with one of the following shortcodes – <code>[SIM-BY-TAG]</code> <code>[SIM-BY-CAT]</code> <code>[SIM-BY-MIX]</code> for single post pages only or for all pages use <code>[SIM-BY-TAG-MULTI]</code> <code>[SIM-BY-CAT-MULTI]</code> or <code>[SIM-BY-MIX-MULTI]</code></li>
<li>To display a Similarity list without altering your templates simply select the function that you would like to auto-display at the bottom of the options page.</li>
</ul>

Similarity

Similarity displays a list of posts similar to the current post with similarity being determined based on tags, categories, or both.

Aaron D. Campbell

There is a <a href="http://bluedogwebservices.com/problem-related-post-plugins/" rel="nofollow ugc">problem 
with related posts plugins</a>, and Efficient Related Posts is fixing that by 
approaching the problem from a different direction and offering a very different 
solution.
Basically, current related post plugins build the list of related posts on the 
fly when the user needs to view it. Since blogs tend to be viewed far more 
often than they are updated (often hundreds of times more often), these queries 
are run way more times than they need to be. This not only wastes CPU cycles, 
but if the queries are slow (which they will be if you have 1000s of posts and 
tags) then the user gets a poor experience from slow page loads.
Efficient Related Posts moves all this effort into the admin section, finding 
related posts when a post is saved rather than when the user views it. The 
advantage is that if the query is slow it happens less often and the post writer 
is the one that waits rather than the user (which I think is WAY better).
There are limitations. For example, since the related posts are stored as post 
meta data, we only store a certain number of them (10 by default, but you can 
set it to whatever you want). This means that if you decide you need to display 
more than 10, you need to have the plugin re-process all posts. I generally 
display up to 5 related posts, but store 10 just in case I decide to display 
more in some areas. Also, since the related posts are calculated when a post is 
saved, manually adding a tag through the database will have no effect on the 
related posts, although I recommend not doing that anyway.

Efficient Related Posts

A related posts plugin that works quickly even with thousands of posts and tags.  Can be added automatically to the end of posts.

Similarity vs Efficient Related Posts