The era of networked computing is definitely old enough to have its own nostalgia today. I still remember using my first real computer -- a Commodore-64 -- and a 300-baud modem to dial up a local BBS and spend hours sharing ASCII files and chatting with other users. (My first actual computer was a rubber-keyed VZ-200, as it was known in Australia, but it didn't connect to a modem.) A few years later I had my first taste of multi-user computers -- it was an old VAX system, I think -- and a whole new world of communicating with machines over the still-young internet opened-up. Everything was done via the command-line, of course, and learning all the commands and tricks to get access was an adventure all in itself.
If this sounds familiar to you, and if world and phrases like "uunet" "rlogin", "yell sysop", "Usenet" "motd" and "finger" invoke fond memories, you'll love Telehack. Telehack is half adventure game, half simulations. It replicates the Internet as it was in the late 80's or early 90's, with hundreds of simulated servers and BBS systems that you can "remotely" log into and investigate using simplified Unix command line tools. You can even find applications to help you "hack" into these simulated servers, and find period-accurate artifacts like "real" users logged in, interesting ASCII files to read, and text-based games to play. In fact, the hacking provides the adventure-game part of the experience, with a series of increasingly-difficult quest tasks to accomplish.
To get started, open a shell and telnet to telehack.com, where you'll see something like this:
There's also a web-based interface, but I recommend using telnet for the appropriate atmosphere. There are fairly detailed instructions available, but half the fun is figuring it all out on your own. Also check out this interview with Forbin, the anonymous game creator.
May the command line live forever.
Telehack.com: About Telehack