Zine was designed to be extensible via plugins. A lot of the functionality provided by Zine is already implemented as a plugin, like the default themes.
In the near future we will add a central plugin repository under this URL. Until then have a look at the Zine wiki for a list of available plugins.
If you are eager and want to start developing your own plugins check out the existing plugins. The “Eric The Fish” plugin is well documented and shows how the plugin interface works.