Z2 - the largest archive and most active community out there

 ZUltimate - an 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. You can find 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.

 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