A door in a bulletin board system is an interface between the BBS software and an external application. The term is also used to refer to the external application, a computer program that runs outside of the main bulletin board program.
Sometimes called external programs or chains, doors have been described as "the 'apps' to the BBS platform." A sysop could differentiate his BBS from others by adding add various doors: games, utilities, or other extensions.
- Strategy games such as TradeWars 2002, Solar Realms Elite, Space Dynasty, and Barren Realms Elite
- Role playing games such as Legend of the Red Dragon, Arrowbridge and Assassin
- Board- and card-style games such as Global War
Other door applications
While many of the most popular BBS doors have been games, numerous doors were utility applications  such as user opinion polls, ANSI art galleries, or the time bank, which permitted users to time-shift their rationed BBS use.
Frequently doors acted as a front-end to themed databases on subject such as astrology, numerology and fortune-telling, recipes, weather prediction, personal ads (sometimes with additional match-making functionality), classified ads and "for sale" listings (sometimes permitting auctions), BBS lists, and parting comments from the most recent BBS callers.
From the 1990s on, most BBS software had the capability to "drop to" doors. Several standards were developed for passing connection and user information to doors; this was usually done with "dropfiles", small binary or text files dropped into known locations in the BBS's file system. Most doors were responsible for operating the serial port or other communications device directly until returning control to the BBS. Later development of FOSSIL drivers have allowed both BBSes and their doors to communicate without being responsible for direct operation of the communications hardware.
- "A Glossary of BBS Terms". InfoWorld: 41. 15 Aug 1988. http://books.google.co.uk/books?id=ZzoEAAAAMBAJ&lpg=PA41&ots=2V6Th3OlY6&dq=BBS%20door&pg=PA41#v=onepage&q=BBS%20door&f=false.
- Andrew Chen (26 Aug 2009). "BBS Door Games: Social Gaming Innovation from the 1980s". Inside Social Games. http://www.insidesocialgames.com/2009/08/26/bbs-door-games-social-gaming-innovation-from-the-1980s/. Retrieved 1 Feb 2013.
- Wolf, Mark J P (2008). The Video Game Explosion: A History from PONG to PlayStation and Beyond. ABC-CLIO. p. 155. http://books.google.co.uk/books?id=XiM0ntMybNwC&lpg=PA155&ots=1WuozltpJh&dq=BBS%20door&pg=PA155#v=onepage&q=BBS%20door&f=false.
- "BBS Door games". Moby Games. http://www.mobygames.com/game-group/bbs-door-games. Retrieved 18 October 2012.
- Edwards, Benj. "The Internet's Forgotten Games". PC Magazine. http://www.pcmag.com/slideshow_viewer/0,3253,l%253D255130%2526a%253D255131%2526po%253D10,00.asp?p=n. Retrieved 18 October 2012.
- Morrow, Blaine Victor (1996). Dial Up!: Gale's Bulletin Board Locator. Gale.
- Tresca, Michael J (2010). The Evolution of Fantasy Role-Playing Games. McFarland. p. 93-94.
- Werner, Ray (1995). BBS Secrets. IDG Books Worldwide, Inc.. p. 597. ISBN 1-56884-491-3.
- BBS Archives - Collection of BBS doors and related files
- BBSFiles.com - door games, utilities, and forums
- Dropfile formats
- BBS doors at the Open Directory Project
Door games on this wiki
- Final Frontier
- Esterian Conquest
- Shadow Board
- Board Master
- BBS Simulator
- Swords of Chaos
- BBS door game
- The Pit
- Murder Motel (PC)
- Land of Devastation
- Operation Overkill II
- Ultimate Universe
- The Arcadian Legends
- Dungeon Master
- Global War
- BBS door
- Falcon's Eye
- VGA Planets
- TradeWars 2002
- Legend of the Red Dragon
- Phantasia (video game)
- Pyroto Mountain
- Category:Major BBS games
- Trade Wars
- STellar Chaos
- Kannons and Katapults
- Space Empire Elite
- Space Dynasty
- Solar Realms Elite
- Planets: The Exploration of Space
- Yankee Trader
- Barren Realms Elite
- Cripple Smash