Grabs related links from Google Blog Search, inserts a link to them into your post and gives them a pingback.
** 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).
Image by Toban Black; some rights reserved. See link for details.