Zoom

Zoom is a Z-Machine, which means that it plays text adventure games written in ZCode. The original games were written by a company called 'Infocom' in the 80's: these are available on the Infocom Masterpieces CD, published by Activision.

Unfortunately, Activision seems to have lost interest in this title - however, there are a few companies that have a stock of this CD (I bought mine from here).
In around 1993, a compiler called Inform was released that allowed ordinary mortals to create their own ZCode adventures. This sparked quite a revival, and there are many games available for download here:
ftp://ftp.ifarchive.org/if-archive/games/infocom/
The Mac OS X version supports all the features of the X-Windows version, has a much better user interface, and supports several extra features, such as speech output.