What was even more amazing is you could daisy chain serial ports on computers to get multiplayer Doom running. One or more of those links could even be a phone modem.
Downside is that your framerate was capped to the person with the slowest computer, and there was always that guy with the 486sx25 who got invited to play.
Someone tried running that in one of the campus computer labs when I was a student, and the (probably misconfigured) IPX routers amplified it into... a campus-wide outage. Seems weird to me, but that's what the big sign on the door said the next day.
