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.

baptiste.gourdin

TalkBack is a open-source secure LinkBack mechanism based on a public-key cryptography that aims at tackling the blog spam problem at its root: instead of detecting spam via the content analysis like Akismet, TalkBack prevents spammers from posting LinkBack notifications. When installed TalkbBack add the two following lines of defense to your blog:
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The first line of defense is a lightweight PKI (Public Key Infrastructure) that ensures the identity of blogs by using public key cryptography. Moreover using cryptography allows TalkBack to guarantee that no one can tampers with your TalkBack notifications and that the receiver will received them unaltered.
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The second line of defense, is a global rate limiting system enforced by the TalkBack authorities that ensures that with a single blog identity, a spammer cannot massively spam TalkBack aware blogs.
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TalkBack is compatible with every other defense mechanism and do not store/disclose any private information about your blog or you. As a matter of fact if you don’t trust the network, TalkBack can also encrypt your notification to protect them from eavesdroppers.
A complete description of Talkback is available here: http://code.google.com/p/talkback-client/downloads/detail?name=talkback-paper.pdf and the source code here: http://code.google.com/p/talkback-client/downloads/detail?name=talkback-wordpress-plugin.pdf

Talkback

TalkBack is a secure protocol for LinkBack notification designed to prevent the blogosphere from being spammed.

Pingcrawl vs Talkback