Zork I 1.0 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The software lies within Games, more precisely Arcade. The most popular version among the application users is 1.0. We recommend checking the downloaded files with any free antivirus. This free software for Mac OS X is an intellectual property of Infocom.
From the developer:
Zork I classic fantasy game set in a magical universe. The ZORK trilogy takes place in the ruins of an ancient empire lying far underground. You, a dauntless treasure-hunter, are venturing into this dangerous land in search of wealth and adventure. As an aspiring adventurer you will undoubtedly want to locate the treasures and deposit them in your trophy case.
You may want to check out more Mac applications, such as Qwak, Gaichu or Plantas VS Zombis, which might be similar to Zork I.