Free look

Unlike Heretic, Hexen, Strife and most modern first-person shooters to date, Doom and Doom II only featured the ability to look around horizontally, or side to side. Free look in FPS games stands for the ability to move the player's view both horizontally and vertically to see not only around the player, but also above and below. Doom originally had an auto-aim feature that directed the player's shots at shootable targets that were above, below, or straight ahead in the player's field of view. When using free look, auto-aim is not needed to shoot at targets that are above or below the player's direct aim, because the player can move the view up and down as needed and pinpoint the target's height manually.

Source ports
After the Doom source code was released, many source ports have been created that have free look as an optional feature.
 * ACE Engine (Through custom code)
 * CDoom
 * csDoom
 * DOOM BFA
 * DoomGL
 * Doom Legacy
 * DoomPC
 * Doomsday
 * DOSDoom
 * EDGE
 * Eternity Engine
 * GLBoom+
 * GZDoom
 * Odamex
 * ReMooD
 * Risen3D
 * Vavoom
 * ZDaemon
 * ZDoom