Roundcube is released with a set of default plugins, these are all maintained by the Roundcube devs. The sysadmin can choose what, if any, of these plugins they'd like to enable by adding them to the 'plugins' array in their config file.
There are 2 plugins 'filesystem_attachments' and 'jqueryui', which are required for Roundcube to function correctly, these plugins will always be enabled whether or not you add them to the config array.
Third party plugins for Roundcube can be found here
https://plugins.roundcube.net and can be installed via Composer.