Nazi Zombies: Portable is a free and open source Call of Duty: Zombies demake - eviltoast
I'm surprised I hadn't heard of this one before! A few developers have been building Nazi Zombies: Portable, a Call of Duty: Zombies demake powered by various enhanced forks of the Quake engine.

“A few developers have been building Nazi Zombies: Portable, a Call of Duty: Zombies demake powered by various enhanced forks of the Quake engine.”