Z2 - the largest archive and most active community out there

 ZUltimate - an informative all-ZZT wiki, hosted at Z2
Anna Anthropy wrote a book about ZZT, its users, and the communities that formed around it as part of a Kickstarter-funded project.

She interviewed several people to help tell the story (including me).

The book was released in 2014, and you can read more about it here or purchase it here.
Other ZZT-related links:

 bitman's ZZT review

 Castle ZZT by Jack Masters

 Exploited Chaos by Viovis

 Interactive Fantasies by Hydra and Hercules.

 7th heaven by Luke Drelick

 Wikipedia - the entry about ZZT

 ZZT++ - an open-source ZZT re-creation

 ZZT Cartography - maps generated from ZZT games

 ZZTDOS by ringworm

The Graveyard:

 barrel remnants by Mono. Formerly the Big Bustin' Barrel

 chain link fence (Roadblock) by tseng

 DreamZZT by Sam Steele - ZZT for Dreamcast?

 eaglerock interactive by viovis

 k05mik m3l-l4wn by t0ucan

 lamegame by stardreamer

 MadTom's Pick

 Norris's Megazeux and ZZT page - the oldest ZZT page?

 Megazealot: Links by Darren Hewer

 mworld by Matt Williams

 official ZZT home page by Tim Sweeney

 the wonderful many worlds of ZZT by barjesse

 ZZT encyclopedia online by chronos

 ZZT archive ( by xabbott