Australian modder "The Zombie Killer" has been heavily involved in the Doom modding community for some time (he published a visual novel engine for ZDoom early this year) and now he's working on a "Classic Arcade" mod that allows you to play games like Wolfenstein 3D and Doom...inside Doom.
The goal is to mod Doom so that you can play a bunch of different arcade cabinets, but the latest version (released yesterday) only supports putting a Wolfenstein 3D cabinet in Doom 2 and a Doom cabinet in the original Doom.
It's sort of fascinating, from a game design perspective, to watch video of the in-progress mod and see the Doom marine playing Doom, then walking up behind himself (both games of Doom coexist on the same level, at least in the mod's current version) and shoot himself in the back.
This isn't the first time a playable game machine has been modded into Doom, of course; the decades-old game has a vibrant modding community, and many have used ACS (Action Code Script) over the years to build playable games inside Doom.