Most of the Infocom games ("The Lurking Horror", "Planetfall", etc.) are *not* legally available on the Internet. They are still under copyright and may be bought in various collections from Activision. "Masterpieces of Infocom" contains all the Infocom games except "The Hitchhiker's Guide to the Galaxy" and "Shogun". There are other out of print collections such as the "Comedy Collection" and the "Sci-Fi Collection". Many Infocom games can be found on auction sites such as <http://www.ebay.com/>.
However, the three "Zork" games are available for free download from the Activision website at <http://www4.activision.com/games/zgi/atrium/gameroom/main2.html>. "The Hitchhiker's Guide to the Galaxy" is available for free Java-based play at <http://www.douglasadams.com/creations/infocomjava.html> and it is simple for one to find the ZCode file on that site.