1994. I used to go to the Univ. of Kansas computing center to play DOOM on the LAN, it usually was the shareware version, and all computers got wiped weekly anyway. I was too young to go to college but I didn't have a computer so I spent a bit of late nights there. Eventually got a copy of the registered version and there were many pick up games to be had. The hours spent designing levels on DoomCAD and playing ledges.wad, T2.wad, etc, was amazing.
One night I was in there alone playing single-player mode and a cop walks in, his mind was blown. He asked if we could watch and spent a couple hours watching me play and I taught him how to play.
I went from being barely able to learn how to install the game from disks to a sysadmin a few years later. DOOM was what kindled my interest in computing in general.