[W]ordPress pluginn repositor blog & website software hosts on their own site, a large collection of 3rd-party Plugin titles for the program. As of late 2010, there are over 15,000.
They call the page for this repository, WordPress Extend-Plugins.
WordPress does some vetting of these plugins; provides a standardized format for organizing information about plugins, and creates a ‘marketplace’ in which folks can ‘shop’ for plugins.
Individuals, organizations and businesses all produce plugins, for a wide variety of reasons. With WordPress involved in the promotion & delivery of plugins, the ‘reasons’ for offering plugins can’t stray too far into territory that you don’t want to inadvertently find yourself in. To have WordPress ‘ride herd’ on the plugin scene, is A Pretty Good Thing.