While not by Infocom, Photopia is probably the best Interactive Fiction I every played.

You can download it at http://www.wurb.com/if/game/255 (http://mirror.ifarchive.org/if-archive/games/zcode/photopia.... probably being the best choice). It's possible to play it online at http://www.ifiction.org/games/play.php?game=332&mode=htm..., but it's not as much fun as with the real version.

To open the .z5 file you need something like Zoom [1] for Mac, or Frotz [2] for Windows.

[1] http://www.logicalshift.demon.co.uk/mac/zoom.html [2] http://frotz.sourceforge.net/