agrafix

This plugin adds hyperlinks to specified words in posts and pages. This can be very usefull if you are using complicated words and want to link Wikipedia articles or 
if you just want to improve your seo by linking some interessting words 🙂
<h3>Example</h3>
Your Text: 
 Welcome to a blog about google
Your Plugin-Settings: 
 blog|Wordpress software|http://wordpress.com 
 google|Search engine|http://google.com
Result: 
 Welcome to a <a href="http://wordpress.com" title="Wordpress software" rel="nofollow ugc">blog</a> about <a href="http://google.com" title="Search engine" rel="nofollow ugc">google</a>

agLinker

This plugin adds links to specified words in posts and pages.

Pranav Rastogi

Manage taxonomies from WordPress back-end. Add/Edit/Delete taxonomies and more!
Features:
<ul>
<li>Create taxonomies.</li>
<li>Delete taxonomies.</li>
<li>Edit taxonomies.</li>
<li>Add taxonomies for custom post types.</li>
<li>Add taxonomies for pages and links.</li>
<li>Widget to display terms of a taxonomy as a tag cloud or list.</li>
</ul>
More features in future versions.
<h3>Usage</h3>
<a href="http://pranav.me/plugins/taxonomy-manager/" title="Taxonomy Manager - Usage" rel="nofollow ugc">Taxonomy Manager – Usage</a>

Taxonomy Manager

Add, Edit, Delete & Manage taxonomies for posts, pages, links and custom post types with a few clicks of mouse. Makes adding taxonomies a 100 time …

agLinker vs Taxonomy Manager