There are quite a few communities and services built around software (notably extensions and plugins). For example,
Firefox and
Opera provide skins and extensions;
WordPress has themes and plugins;
Drupal is essentially is built for extensibility; and
Eclipse has just about anything you can build on top of the platform. Extensibility alone is a major factor in the success of many open source projects.
And then there are behemoths like
Ubuntu and
PCLinuxOS and
Directory Opus, each of whose communities contribute a large share to their success.
So which software apps/projects have the best supported and most vibrant communities?