Zork: The Undiscovered Underground

It's time to get back underground
Zork: The Undiscovered Underground is a text-adventure game released for free by Activision in 1997 as a prequel and promotional item for their upcoming graphical adventure Zork: Grand Inquisitor.
 
It was written by former Infocom "Implementors" Mark Blank and Michael Berlyn. 
 
Since it was written in the Inform programming language, it can be run on any platform that supports a Z-Machine Interpreter.