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.

majick777

Pingchecker is a free plugin for WordPress that allows you to scan your post’s content for 
links, check the pingability of those resources you’ve linked to, and manually ping those 
pages. This improves upon the inbuilt fuctionality of WordPress by allowing you to receive 
the results of your attempted pings whereas WordPress doesn’t. (With WordPress your ping 
either appears in the trackback list or it doesn’t, with no explanation or error codes.)
Also included is a workaround for a bug in the WordPress XML RPC server that prevents many 
of your pingbacks from succeeding without you even knowing about it! When you ping another 
blogs server, it will check the page you linked, BUT because of this bug, sometimes it can’t 
find the link at all. This workaround adds a hidden div to your footer with your links so 
they can be found, greatly improving your chances of a successful ping.
<h3>Recommended Use</h3>
<ol>
<li>Before publishing your post, use Pingchecker to check the pingability of the resources 
you are linking to. If they aren’t, you may wish to choose alternative similar resources 
that are pingable instead.</li>
<li>Then, publish your post and WordPress will attempt to ping the resources automatically 
as it normally would. Check the trackback list under your content box to see if your ping 
was successful as usual.</li>
<li>If the new trackback/pingback does not appear, use Pingchecker to ping the resource 
instead. The results of your attempted pings will be returned in an alert box.</li>
</ol>
<h3>The XML RPC Server Bug Workaround</h3>
While working on this plugin I noticed a large occurrence of the pingback fault 17: 
“The source URL does not contain a link to the target URL, and so cannot be used as a source.” 
A really frustrating message given you are sending a pingback because the source DOES contain 
a link to the target, yes? Might I point out that very few WordPress users are aware of this 
even happening because nowhere does WordPress actually return you these fault codes..!
Well, after a bit of testing I found the bug seems to be in the XML RPC server code for 
Wordpress, specifically the strip_tags function in PHP is just not reliable enough for getting 
anchor links on the variety of WordPress templates out there (IMHO). (Line 3422 in WP3.1)
Unfortunately, since the bug is in the server code itself, you can’t fix it on someone elses 
blog can you? That’s why this is a workaround instead. The Pingchecker workaround will scan 
your post content for links using regex instead, then echo a hidden div element containing 
all the links (with an added nofollow tag so you aren’t linking twice) in your blogs footer, 
which is picked up much more easily by the strip_tags function in use by the server.

Pingchecker

Scans post for links, checks if they are pingeable and sends pingbacks with results returned, improves chances of successful pings!

Pingcrawl vs Pingchecker