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.

Ryan

Adds a shortcode, series-tag, that you can use to create a list of all posts 
that are tagged with whatever tag you pass in.
Series Tag was written to allow you to tag a series of posts with the same tag, 
and then easily link between them without having to manually create links at the 
bottom of each post.
Links are created in date published order starting at the earliest date. All post 
titles are links to the permalink of the post except for the current post which just has its title displayed.
For help or more info, visit <a href="http://www.nutt.net/tag/series-tag/" rel="nofollow ugc">Nutt.net</a>

Series Tag

Adds a shortcode so you can easily create a list of all posts with a specific tag. Written to make it easy to link between a series of posts.

Manual Related Links vs Series Tag