Host software is software that runs on the host computer (typically a PC style computer, but could be any programmable device with a USB host controller).
opcd_test is a small tool for tweaking individual bits of OpenPCD from the command line. It is available from http://svn.openpcd.org/trunk/host/.
librfid is a library that implements the reader side of common RFID protocols such as ISO 14443, ISO 15693. librfid is not part of the OpenPCD project, even though the same people wrote it. librfid contains support for the main_dumbreader firmware.
You can obtain librfid from http://svn.gnumonks.org/trunk/librfid/.
For some basic information on how to get started using librfid, please see Getting_started#Using_librfid.
For tuning of the mod_conductance value for ISO 14443-B with librfid see here.
In development. This will provide a scriptable shell-style interface to the OpenPCD reader.