Language is evolving and occasionally we need to come up with a new name for something. The name 'contributed plugins' is rather misleading, as it suggests that there are plugins which are not contributed; also ALL code may be considered as contributed to the Moodle project. What we are referring to is code contributed to Moodle which is not included in the standard Moodle download package.
From looking at how other projects name such code, and from discussions at the hackfest last October, we've come up with a new name for contributed plugins: Add-ons.
A summary (from Martin):
Moodle = core code + plugins
Plugins may be either standard plugins (included in the Moodle download package) or add-ons.
Thus this forum will be renamed accordingly, together with other forums for discussing add-ons.