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.

Andy Charrington

Easily display page specific links on any page or post by quickly adding a link directly from the add/edit page/post screen.
Delete links by clicking the trashcan next to the link.
Supports adding URL, name, target and description.

Add Links to Pages

Add Links to Pages allows you to add page specific links and have tim displayed through a widget.

agLinker vs Add Links to Pages