Games (almost exclusively text-based
interactive fiction) developed by
Infocom between 1977 (when the first mainframe
Zork originated at
MIT) and 1987 (although some games have borne the Infocom label since then, as
Activision hold the rights). These games are unique and certainly ahead of their time from a technical vantage point.
An Infocom adventure consists of several components. The core is a "story" file, which is effectively a ROM for a hypothetical "adventure game interpreter" called the ZMachine. Implementations of the Z-Machine (interpreters) exist for about 10 8- and 16-bit microcomputer platforms that were used commercially, with many more (including several Java versions, as well as one for the Game Boy and the Palm) being developed by enthusiasts. The Z-Machine is a virtual machine with a CPU (which reads z-codes), two output devices (screen and printer), and a memory store. Part of the memory can be mapped out to disk, making this the first known implementation of virtual memory (yes, anywhere). There are many oddities in the Z-Machine system. For instance, three characters are stored per byte. There were 8 revisions of the system, increasing the maximum possible world size, and adding some rudimentary sound and graphics. Due to its exceptionally open-ended nature, it has been used for many things : including a catalogue of interactive fiction games. Many hobbyists use a third party development system called Inform.
As well as the data file and interpreter, Infocom games also came packaged with a large range of freebies, ranging from full-colour manuals to comic books, photographs, maps, code sheets, small pieces of fluff and glow in the dark plastic magic stones. These served two purposes : obviously, a veiled copy protection mechanism (bolstered by the fact that the neato toys were a large part of the incentive for buying the game!), and secondly, to provide atmosphere (especially the case with Deadline, which was apparently designed for social play ... N64 style). I would estimate that about two thirds of the 40 or so titles that were released cannot be completed without access to the in-box materials -- or at least scans and transcripts thereof.
The ZMachine and the games it powered were profoundly influential on adventure game design. LucasArts' SCUMM system, and Sierra's equivalent tools have some similarities in design.
The latest platform to be graced by the Infocom canon is WAP. Activision and Nokia are hosting (initially 12) online versions of Infocom classics. There are all-new hint systems, and loading and saving on the server. A boffo frob.
For more information (than you could possibly need) about these games : http://www.crosswinds.net/~dsinclair/index.html