Content.ad

<a href="http://www.content.ad" rel="nofollow ugc">Content.ad</a> is a content recommendation platform that helps site owners generate more time-on-site and monetize their content easily and unobtrusively. Content.ad serves billions of impressions across thousands of sites every month. The WordPress plugin is the quickest and easiest way to get Content.ad up and running on your site.
<h4>Key Features</h4>
Simple Setup – Our 3-step guided installation gets you creating your first widget in minutes.
Flexible Styling – Our widget creation wizard makes customizing the look and feel of the widget simple and straightforward.
Performance Driven – Our state-of-the-art algorithms learn what content is most popular with your users, lowering bounce rates and increasing overall site revenue. Also, since optimization takes place on Content.ad servers and the widget is loaded asynchronously, the plugin doesn’t slow down your site.
Powerful Options – Free enterprise-level features let you control how the widget integrates into your site and what kind of content appears in it, all without leaving your WordPress admin.
Rich Reporting – Use our robust reporting and graphing options to see exactly how well your widgets perform down to the hour.
Personalized Support – Our US-based support staff is available to help you properly set up and optimize Content.ad to meet your needs.
To learn more about Content.ad, visit our <a href="http://help.content.ad/" rel="nofollow ugc">publisher FAQ</a> or <a href="https://www.content.ad" rel="nofollow ugc">visit our website</a>.

Display popular content to your users from your own site and/or from our sponsored partners, increasing visitor engagement and revenue.

husani

Searches various data sources (Google Blog Search and Twitter) for posts related to your content and displays links/summaries on your page or 
inside your posts. These links and summaries can be displayed anywhere and any way you wish — below your post, in a sidebar, etc. With the 
magic animation power of jQuery, SMIRC content can be hidden until a user clicks the customizable header.
SMIRC performs related content searching (at least in this version) by creating RSS feed URLs for the various data sources. The RSS feed is 
parsed and converted into XHTML for display in your blog.
All PHP is separated into logical classes and files, and all XHTML is standards-compliant. Lists of links, authors, and summaries are built 
using standard UL and LI tags, and can be modified to fit your theme via two CSS files contained within this plugin.
SMIRC is released to the WordPress community under the GPL. Please feel free to modify as you see fit, and if you find this plugin useful, donate 
to the author. All feedback is welcome at wordpressplugins@husani.com, and you can visit the author’s websites at http://www.husani.com and 
http://www.evb.com.
UPDATE FOR v1.1: Fixed a bug with setting header text.

Social Media Integrated Related Content (SMIRC)

Like trackbacks, but not -- this plugin searches and displays content related to your CONTENT, not just posts that link to yours.

Content.ad vs Social Media Integrated Related Content (SMIRC)