For the Amazon_store Drupal module I've been working on for Alonovo.com, I set up several of the pages to use Panels plugins. But there isn't a lot of documentation on how to do this, so I had to dive in and learn it from the examples available in the code. The result was a set of examples of how an drupal module can implement panes, the Panels Plugin Example which is now available on on Drupal.org. There are examples of content_type, argument, context, and relationship plugins. I'm hoping that others will review the code and improve it.
I finally got the Ctools Plugin Example module and documentation updated and submitted. Merlin of Chaos seems happy with the idea, so I'm hoping it can make it as a module distributed with Ctools, which is a fundamental part of Panels.
One of the nicest things about Panels (now provided by Ctools) is the ability to write relatively simple pieces that extend it. But it's been kind of hard to figure out the magic. Hopefully this module + documentation will help out.
Update 19 August: It's been committed, so it will be in the upcoming full release of CTools.