Xonotic - Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux.
DayZ - DayZ is an open world multiplayer survival game that tosses you in the midst of a zombie apocalypse.
Unreal Tournament - Unreal is a series of first-person shooter video games developed by Epic Games.
7 Days to Die - 7 Days to Die is a post-apocalyptic survival game, during which players must craft shelters and horde resources in order to survive waves of undead enemies. Read more about 7 Days to Die.
Quake - Quake is a series of first-person shooter video games released by id Software.
Project Zomboid - The Ultimate Zombie Survival RPG.