Aaron D. Campbell

This plugin allows you to manually enter links that are related to a post. They 
can be on any site and you can enter just the URL or an entire link (specifying 
the title attribute, onclicks, etc). There are helper functions to display them 
in your theme or you can simply tell the plugin to automatically add them to 
posts or even just single posts. The list is output as an unordered list (ul) 
with an optional title, and everything has CSS classes to make styling easy. 
Requires PHP5.

Manual Related Links

A related links plugin that allows you to manually specify links, even if they're on another site. Requires PHP5.

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.

Manual Related Links vs upPrev