You can drop plugins (.jar files) here or move them to another directory and set

    # Linux, Mac OS X &c.
    plugins = /path/to/plugins/dir

or:

    # Windows
    plugins = C:\\Path\\To\\Plugins\\Dir

in PMS.conf

You can find a list of available plugins here: http://www.ps3mediaserver.org/forum/viewtopic.php?f=12&t=9078

For information on developing plugins, see https://github.com/ps3mediaserver/ps3mediaserver/tree/master/src/main/external-resources/plugins/README.md