They were using NeXT. id was all next at the time of DOOM. The best way to track id's tech stack is to look at their level design tools: -DoomEd (not the one

They were using NeXT. id was all next at the time of DOOM. The best way to track id's tech stack is to look at their level design tools: -DoomEd (not the one you're thinking of - it never made it beyond id) was for NeXT -QuakeEd - It was for NeXT: You can grab the code at http://www.gamers.org/dEngine/quake/QuakeEd/source.html http://www.gamers.org/dEngine/quake/QuakeEd/source.html. If someone has the know-how, they might be able to update it for GNUSTEP or OSX, but it's useless in its current form. -QE4 (QuakeEd v4) - id's internal Quake 2 editor. It marks the shift from NeXT to WinNT. https://github.com/id-Software/Quake-2-Tools https://github.com/id-Software/Quake-2-Tools -Q3Radiant - Robert Duffy got sick of the bad parts of QE4, and modified it into QERadiant. It was all GPLed, anyways. Then he got hired (I think) by id - QERadiant formed the base for Quake 3's level editing environment - Q3Radiant. GTKRadiant - GTKRadiant was (and still is) maintained by Timothee Besset, who was also responsible for the Quake3 linux ports. It was a (soft) fork of Q3Radiant made engine-independent, and was also based on GTK, so it could support Mac and Linux. It was dual-licensed by id (it was an internal project - god knows why) until the rest of Q3 opened up. It's also the only editor on this list still in active development (as is its fork, NetRadiant). Who knows what they're using internally now: id has closed their doors since the acquisition.