A BBS door was a mechanism to execute and communicate with an external program, commonly a game on bulletin board systems (commonly referred to as Door games). The BBS software would start the external program, and the door system would pass data back and forth between the door program, the BBS, and the remote user. To supply the door program with the user's information (such as the user's alias and the amount of time they had spent online), the BBS software would create a drop file containing information for the program to read. Fossil and later Fossil32 were common door systems.

Popular door games included:

• Arrowbridge II
• Barren Realms Elite (BRE)
• Cripple Smash
• Exitilus
• Food Fight (BBS door)
• Global War (game)
• Land of Devastation (LOD)
• Legend of the Red Dragon (LORD)
• Mutants
• Operation Overkill
• PimpWars
• The Pit
• Planets: The Exploration of Space
• Space Dynasty
• TradeWars 2002
• Usurper
• Voyeur
• Yankee Trader