Wikipedia
RBBS-PC
RBBS-PC ( acronym for Remote Bulletin Board System for the Personal Computer) was a public domain, open source BBS software program. It was written entirely in BASIC by a large team of people, starting with Russell Lane and then later enhanced by Tom Mack and including Ken Goosens and others.
It supported messaging conferences, questionnaires, doors (through the DOORDEF.DAT dropfile), and much more.