What's left of Infocom has made the three "Zork" games available as free downloads,
here, if you don't already have them.
"Hitchhiker's Guide to the Galaxy" is available from Douglas Adams' web site. You may need to jump through the hoops at
the entry page, and enter the serial number to get access to the next directory, but once you've done that, the actual game file is at
http://www.douglasadams.com/creations/hhgg.z3 (right-click and save to disc).