Best free "first-person shooter" games

GAME GENRE – First-person shooters (FPS) are video games played from a first-person perspective, where players shoot enemies to progress through levels. Multiplayer FPS often focus on arena-style combat, with players competing in “kill-everyone” matches without levels or storylines.
Popular non-free first-person shooters include Doom, Half-Life, Quake, and Halo.