A very popular BBS software.
BBBS was written by two Finns, Kim "B" Heino and Tapani T. "Z" Salmi, and has been in development since 1990. It is distributed as shareware. (For small BBSes, the prices are very modest, though.)
It supports "traditional" modem/ISDN BBSing, Telnet, and even WWW-based use. It supports Fidonet (and other similiar networks) and also wide range of Internet protocols (including NNTP, also shown in the number of Usenet posts coming from Finnish BBSes.) The full feature list is too large to mention; Let's just say it supports everything every BBS has ever supported. It also features a powerful "BZ" scripting language.
BBBS has been ported to wide range of platforms: MS-DOS, OS/2, Windows, Amiga, Linux and FreeBSD.
Home page: <URL: http://www.bbbs.net/ > - also has a list of some public BBBS-powered telnetable BBSes.