They even wrote their original level editors, DoomEd and QuakeEd in Objective-C! Back in 1993 this pre-Cocoa based application allowed them to edit the same DOOM levels simultaneously over their network:
In fact, with the superpower of NeXTSTEP, one of the earliest incarnations of DoomEd had Carmack in his office, me in my office, DoomEd running on both our computers and both of us editing one map together at the same time. I could see John moving entities around on my screen as I drew new walls. Shared memory spaces and distributed objects. Pure magic.
How cool is that!
found via: Tiny Subversions