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. Inform itself has a homepage here
This directory contains binary versions of Zoom compiled for M$ Windows. The source tree is the same, so if you want to hack on it, go here. You'll need a recent version of cygwin if you're compiling under Windows, or a mingw32 cross-compiler if you want to compile under a real operating system.
If you just want to play the games, just download the zip file corresponding to the version of Zoom you want. Zoom is distributed under the terms of the GNU General Public Licence.