Resources

Purchasing Quake / Quake2 / Quake3 / Hexen2

Quake on Steam (expansions separate)
Quake on GoG (with expansions)

Community

func_msgboard
InsideQC
QuakeOne
Quaddicted
QuakeWorld.nu
QuakeWorld Discord
FTE's IRC channel
QuakeC Development IRC channel

Tools

TrenchBroom (map editor)
Tyrutils-Ericw (map compiler)
IQM compiler

Misc Documentation

Particles
Spike's CSQC Primer
Eukara's CSQC Tutorials

Example Mods

The Wastes
Chilling Spree
(OLD)Chilling Spree
FreeCS
PureCSQC
Pourtall
Chillo's Wee2
Shpuld's Mods in Your Browser Mobster Massacre
Treasure of the Hellbeast
Human Complex
Gardening Sim 2019
Escape From Space (Official Site)
Siege at SPC (Official Site)
Regain Power (Official Site)
Bloodletter (Official Site)
Shoot Your Goo (Official Site)
Jumps Done Well (Official Site)
Issun Boushi (Official Site)
Descendants of the Hellbeast (Official Site)

Other Engines

QuakeSpasm (standard)
QuakeSpasm (spiked/enhanced)
FodQuake
ezQuake
DarkPlaces
... many others