You can now download the source code for all Infocom text adventure classics - Yes, The Hitchhiker's Guide to the Galaxy and Zork are both included.

The source code for all classic Infocom text adventures, including The Hitchhiker’s Guide to the Galaxy and Zork, is now available for download. Infocom originally designed these games to run on a virtual machine called the Z-machine to support diverse computer architectures, and interpreters for it exist on many modern platforms. The interactive fiction community continues to create games using ZIL and the Z-machine, alongside newer tools like Twine and Ink. Although Activision retains rights and could request takedowns, the source code release benefits historians, narrative designers, and game developers.