Games similar to Doom include Half-Life. It's paid, but Xonotic and AssaultCube are free. Other options are Quake, Fallout, GZDoom and Shadow Warrior (Reboot). First-Person Shooters like Doom are a crowded field, with more than 50 alternatives for Windows, Linux, Mac, Android and the web.
Freedoom is a WIP a complete free content game based on the Doom engine. It's not an engine, but it's replacement game data with entirely original content, Compatible with a majority of game mods made for Doom. Comes in Three Flavors~.
1998 saw the public release of the Doom source code by its original authors, id Software. Since then, a large number of Doom "source ports" have been created by programmers, who have extended the code, adding new features and removing some of the limitations of DOS...
Zandronum is a multiplayer oriented port, based off Skulltag, for Doom and Doom II by id Software.
OpenArena is a community-produced deathmatch FPS based on GPL idTech3 technology. There are many game types supported including Free For All, Capture The Flag, Domination, Overload, Harvester, and more. There are 17 unique player models to choose from and 12 weapons.
United States
Prey casts players aboard space station Talos I in the midst of an experiment gone awry. The station is overrun by hostile aliens and players must survive using mind-bending powers and weaponry as the fate of humanity is at hand.
United States
Fight under the onslaught of mutated monsters, obsessed soldiers and even darker bosses. Just try to survive and find out what to do next in this life, simultaneously tearing your enemies to pieces.
Project Warlock is a first person shooter that fans of Doom, Hexen and Wolfenstein cannot miss. It serves you an exploding cocktail of bullets, spells and monsters.
Poland
EU
Developed by id® Software, the creators of DOOM® and QUAKE®, RAGE is a groundbreaking first-person shooter set in the not-too-distant future after an asteroid impacts Earth, leaving a ravaged world behind.
United States
Dead Space is a survival horror third-person shooter video game for Microsoft Windows, PlayStation 3 and Xbox 360.
United States
Set in the iconic Alien universe, Aliens: Fireteam Elite is a cooperative third-person survival shooter that drops your fireteam of hardened marines into a desperate fight to contain the evolving Xenomorph threat.
United States
Aliens vs. Predator is a science fiction first-person shooter video game, developed by Rebellion Developments
Japan
Hard Reset is modeled on "old school" video games such as Quake and Unreal, which results in more straightforward gameplay than most modern first-person shooters. The various stages have secret areas with hidden pick ups such as health and ammunition.
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