farinspace

This plugin allows you to show and hide specific portions of content within a post or page. Visitors can then use custom show/hide links to show or hide the portions you’ve defined. The plugin enables a few WordPress shortcodes: <code>[peekaboo]</code>, <code>[peekaboo_link]</code> and <code>[peekaboo_content]</code>.
To learn more read the <a href="http://farinspace.com/wordpress-peekaboo-plugin/" title="Peekaboo extended documentation" rel="nofollow ugc">extended documentation</a>.

Peekaboo

This plugin allows you to show and hide specific portions of content within a post or page. Visitors can then use custom show/hide links to show or hi …

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.

Peekaboo vs agLinker