This addon makes it easier to add a Yahoo! Pingbox to your Joomla instance
#Update
As of 12/2012, Yahoo! has discontinued the pingbox functionality in Yahoo! Messenger. This plugin is no longer functional.
I leave it here as a reference for creating other Joomla plugins.
http://lehmoine.org/joomla15/programming/the-web/150-add-a-yahoo-pingbox-to-your-site.html