Add yours, add categories, link everything!

Mapping, Gaming, Singleplayer, Design: http://www.celephais.net/board/

Modding, Coding: http://www.inside3d.com/

NetQuake: http://www.quakeone.com

QuakeWorld, Multiplayer: http://www.quakeworld.nu/

News aggregation: http://www.planetquake.gamespy.com/

Non-English:

Germany: http://www.quake.de/

Brasil: http://www.quakebrasil.com.br

Czech: http://www.quake.cz/ (Quakeworld and other Quake titles)

All console variables in DarkPlaces: http://www.quakewiki.net/darkplaces-wiki/console-variables/

Quakeguy (FifthElephant)'s tumblr: http://quakeguy.tumblr.com/

Crash: http://www.quake2.com/crash/reviewsplash.htm

Daz (Custom Gamer) on youtube:http://www.youtube.com/user/darrenweekes

Daz's Quake 1 reviews: http://www.youtube.com/playlist?list=PLB827907AD18E9966

[Kona]: Ratings: http://www.electricescape.com/etherealhell/index.php/ratings/ratings-quake-levels

[Kona]: Reviews (occasionally feat. Tronyn): http://www.electricescape.com/etherealhell/index.php/reviews/quake-reviews

TEAMShambler: https://www.quaddicted.com/webarchive/teamshambler.planetquake.gamespy.com/

Underworldfan (occasionally feat. Tronyn): http://www.quakewiki.net/archives/underworld/

Texturing by Metlslime: http://www.celephais.net/stuff/texturefaq.htm

Quake 3 Arena: http://lvlworld.com/

Doom: http://www.doomworld.com/idgames/

Sites that go by the name "Quake Wiki":

• This one has plenty of basic information (weapons, monsters, etc.) about Quake 1, Quake 2, Quake 3 Arena and Quake 4: http://quake.wikia.com/wiki/Quake_Wikia

• This one is solely focussed on Quake 1. A bit lacking in content, but has some nice introductory information for those interested in the technical side of the game (e.g. beginner mappers and modders): http://quakewiki.org/wiki/Main_Page

• This one seems like a cross between a wiki and a Quaddicted-like repository of mods and maps: http://www.quakewiki.net/

Doom wiki (which has plenty of information relevant to Quake as well):
http://doom.wikia.com/wiki/Entryway