One of my pet projects for awhile has been to get the Atari 8-bit version of
the PLATO software (cartridge) working with the cyber1 system mentioned
here.
I've seen some screenshots of it running on Atari 8-bit, it looks pretty
neat. You can also get programs for Windows/UNIX/Mac OS X to connect to it,
which is what I've used so far. I have to say I have a hard time believeing
an instructional tool like this existed so long ago. I was/am very
impressed.