PLATO was a very early, innovative computer-based learning system developed at the University of Illinois; it ran on mainframes, driving large numbers of interactive terminals. (In the late 1960s, custom plasma monochrome bit-mapped displays were produced for it, to allow graphics to be used.) It later became a product for Control Data Corporation, the builders of the mainframes on which it ran (at one point, a CDC 6600).
PLATO also birthed very early examples of many things which later became key elements of online communities, including instant messaging, email, message boards, chat rooms, and multi-player video games. The exact import of these is unclear, since many other early system had similar features, e.g. CTSS had email. While PLATO did not directly influence the early ARPANET community, very large numbers of users were exposed to PLATO, and so its ideas had a means to be adopted elsewhere.
- Brian Dear, "The Friendly Orange Glow: The Untold Story of the PLATO System and the Dawn of Cyberculture", Pantheon, New York, 2017 - A detailed, well-researched history; short of technical detail in some areas, though