Quake's coop is often overlooked since deathmatch was so popular and most mods were competitive in nature. But all of Quake 1 is fully playable in coop, and with a huge number of players even. The official expansion packs are mostly coop-friendly, but I remember having to cheat a couple times to get past some single-player-only traps.
One of the first full 3D first person shooters.More Info @ PCGamingWiki
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