Out of curiosity I'd like to know each game or genre main community site -and others related to video games :

FPS :
-Quake/fast FPS : Obviously here
-QuakeWorld : QuakeWorld.nu
-Quake 2 : Q2scene, tastyspleen.net (NA)
-War§ow : Warsow.net, Warsow-NA.net (NA)
-Unreal Tournament : UnrealNorth
-TF2 : ETF2L, Vanilla TF2, CommunityFortress (NA)
-Wolfenstein ET/RtCW : Crossfire.nu, GamesTV.org (Replays and ETTV)
-CS 1.6 : HLTV.org, GotFrag
-CS:S : Cadred.org
-CoD 4 : TEK9 Networks
-Armed Assault : Armaholic

RTS :
-RTS : GameReplays.org
-Starcraft : Team Liquid, GosuGamers
-Warcraft 3 : WCReplays
-DotA : GosuGamers DotA
-Heroes of Newerth : HONCAST.COM
-League of Legend : Leaguecraft

Others :
-RPG : RPG codex
-Text-based MUDs : The Mud Connector
-Oblivion : The Elder Scrolls Nexus
-Fallout 3 : Fallout 3 Nexus
-Adventure games/Point & click : Adventure Gamers
-MMO : Massively,
-WoW : MMO-Champion, WoW.com, Elitist Jerks
-Flight simulations : AVSIM Online
-Racing : SimRacingWorld
-Mechwarrior : MekTek Development

Arcade - Consoles :
-Fighting games : Shoryuken
-Virtua Fighter : VFDC
-The King of Fighters : Orochinagi
-Tekken : Tekken Zaibatsu
-Shoot Them Up : ShMUPS
-Tetris : Tetrisconcept
-Jazz Jackrabbit : Jazz 2 Online
-Machines/Brands : JAMMA+ (Arcade), Arcade Otaku (Arcade, needs registration), NintendoAGE

Video games related :
-Streaming : Games Cast TV (Esport), H2kTV (Esport), Livestream (General), USTREAM (General), Tastyspleen (Q2, Quake-like), Level Up TV (QL), Holysh1t.net (QL), Quake Live TV (QL), GLHF.tv (SC2)
-Speedruns/High scores : Speed Demos Archive, TASVideos (CheatedTool assisted), Twin Galaxies (Console/Arcade/MAME), MARP (MAME), HARP (All other emulators), Classic Arcade Gaming
-Gaming movies : own-age.com, own3D
-Replays/Demos : CyberFight.Org
-eSport : Rakaka (Swedish), readmore.de (German), Fragster (German, QL-CS-SC-DotA), Against All Authority (French), ...
-Mapping : Func_Msgboard
-Modding : Mod DB
-Demoscene : Pouėt, Scene.org
-Keyboards/mouses : geekhack forums

Etc, I'll add your entries.

N. B. "NA" goes for "North America".