Have you ever wondered where the various things that we take for granted these days came from? Well, I THINK I've found it in PLATO, a service that started out as an academic tool but quickly became the testing ground for various important and fun things. A bunch of people are running a recreation of this with lots of the original content, so you can easily do things like play the first ever MMORPG, use the first forums, etc.
I've quite enjoyed it and I suspect that any other retro fan will too. And you basically get to use an emulated Cray (or at least an emulator of a Cray-designed mainframe)
http://www.cyber1.org/