This Related Posts Plugin for WordPress displays links to a given number of Posts that are related to the current post(s) by Tags or Categories. It can Automatically add links to Related Posts at the bottom of every Post or you can drop the Widget on your sidebar.
Settings: Excluding posts, categories, and tags; section heading; number of links; display post date; and excerpts.

EZ Related Posts Footer Links and Widget

Display a linked list of related Posts by Tags or Categories at the bottom of every post or on the sidebar. Options to show Post Date and Excerpts.

piffpaffpuff

Related Links gives you the possibility to manually link other posts to your current post. But you can also link pages, media or any custom post-type. And in addition you can use custom urls to link to external files. The plugin adds a Metabox to the writing page with a list of all available content.
Features:
<ul>
<li>Shows a list of all available content in a Metabox on the writing page</li>
<li>Multiple links can be selected</li>
<li>Link order can be changed through drag and drop</li>
<li>Custom URLs can be added</li>
<li>Search field to quickly find a link</li>
<li>Simple theme integration with an included widget or <code>related_links()</code></li>
<li>Works with custom post-types</li>
<li>Settings to enable the post-types that should be shown in the Metabox</li>
</ul>

EZ Related Posts Footer Links and Widget vs Related Links