Below you'll find the prequel to Zork Grand Inquisitor, as well as the original Zork trilogy. Each adventure contains its own interpreter, so all you have to do is create a folder for the games, download the file, and run the executable. For Zork: The Undiscovered Underground, simply extract the ZIP file into a directory.

Zork: The Undiscovered Underground
Zork I
Zork II
Zork III

Links

The New Zork Hotel
The official Zork Grand Inquisitor site is done in the fashion of old-time text adventures

Zork Grand Inquisitor Designer Diary
Updates on the game's progress from Director Laird Malamed

Zork Nemesis
The GameSpot review of the previous Zork chapter

Infocom Classic Text Masterpieces
Almost every Infocom game in one collection, including the Zork series and the Enchanter trilogy

Zork Grand Inquisitor
Fan site with news, files, and background information about ZGI

Infocom
Unofficial Infocom site with exhaustive information about the company, the games, and the history of the Great Underground Empire

Encyclopedia Frobozzica
Online version of the definitive guide to the Zork universe

SPAG Archive
FTP server with issues of SPAG, a free informational guide to text adventures from the Society for the Promotion of Adventure Games

Eidetic
Game development company run by Marc Blank, one of the original creators of Zork

Boffo Games
Home to a few ex-Infocom employees, including Steve Meretsky, author of Zork Zero: The Revenge of Megaboz