I attended the University of Illinois at Urbana-Champaign in the early 1980s and I flunked out of college in part because of spending too much time playing games on PLATO, particularly a MMO dungeon game called Avatar. The way things worked, the "free" (i.e. not connected to coursework) account I had could only be used at night. As a result, I and similar Avatar addicts would gather in the basement computer lab on Friday night and play until around 5AM or so, when the system went offline for maintenance. At that point we would go to IHOP for breakfast, then return at 6AM to play another couple of hours, until our accounts were booted off at 8AM.
Strangely enough, this was not conducive to good study habits! Luckily, after I flunked out, I managed to get accepted into another university which did NOT use PLATO!
You can install software that emulates a PLATO terminal, allowing you to connect to a PLATO host (Cyber1.org).
Here's a video introduction to cyber1.org: http://www.youtube.com/watch?v=DgMG9NCWoaU
And here's a video showing a battle in Empire (a Star Trek space battle game): http://www.youtube.com/watch?v=vMPC1eG5cko
You'll need to view these videos large to really see what's happening.