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.

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.

EZ Related Posts Footer Links and Widget vs upPrev