mikegogulski

Pingcrawl aims to replicate WordPress.com’s “Possibly related posts” capability. It queries Google Blog Search for related blog posts (based on tags, or the post title if there are no tags), tries to verify that they are pingback/trackback compatible sites and, if so, adds links to such posts to the end of each blog post where it is invoked.
Scope-limiting configuration options are included to enable Pingcrawl to fit any WordPress installation’s memory/execution time budget.
<h3>Licence</h3>
Pingcrawl is free software released under the GPL. If you find it valuable, you’re invited to make a <a href="http://www.nostate.com/support-nostatecom/" title="Donate with PayPal, MoneyBookers, etc." rel="nofollow ugc">small donation</a> toward its ongoing support and maintenance.

Pingcrawl

Pingcrawl adds a "Possibly related posts" capability by querying Google Blog Search for post tags and pinging the results.

Gabriel Koen

** This plugin now works! ** It broke shortly after I originally uploaded it last year. People are still downloading it, so I decided to make it work. I revamped it and made it totally compatible with the latest version of WordPress.
External Related Posts takes the category, tags or custom taxonomy from your post (or custom post type) and searches Google Blog Search for related posts. External Related Posts then adds a link to that blog to your post and pings them, letting them know you made a relevant post on their subject.
Controls exist for how many blogs to search for, whether to only post links to blogs that accept pingbacks, how many pingbacks to perform, and a randomizer for how often External Related Posts activates (e.g., every post, 50% of your posts, 25% of your posts).
Github: https://github.com/mintindeed/external-related-posts
WordPress.org: https://wordpress.org/extend/plugins/external-related-posts/
Image by <a href="http://www.flickr.com/photos/tobanblack/3773116901/" rel="nofollow ugc">Toban Black</a>; some rights reserved. See link for details.
<h3>Licence</h3>
External Related Posts is released under GPL v2.

External Related Posts

Grabs related links from Google Blog Search, inserts a link to them into your post and gives them a pingback.

Pingcrawl vs External Related Posts