Shareware

Shareware is a software marketing concept where an incomplete or time-limited version of a program is released free of charge or for a nominal distribution fee, with encouragement to further distribute, in order to entice people into buying the full (registered) version. Doom, Heretic, and Hexen were all initially released as shareware, with Doom and Heretic both containing only the first respective nine-level episode in the unregistered version: Doom: Knee-Deep in the Dead and Heretic: City of the Damned. Later, all three games in the shareware release were brought together in a pack called the "Doom Triple Pack," where the player gets to select Doom, Heretic, or Hexen to play in the shareware release.

Part of the reason Doom II was not given a shareware release, according to John Carmack, was that "[a] lot of people consider themselves to have 'finished DOOM' when they just finished the shareware episode."