Zip file containing the Conscientious Objector mod. Unzip into your Doom directory and use the Mod option at the bottom of the main Doom menu screen.
Conscientious Objector is a Doom 3 mod created by thechineseroom, a research project building experimental FPS games based at the University of Portsmouth, UK.
It uses the first three levels of the commercial game, stripped bare and repopulated, together with some custom code that stops the player from being able to kill anything. Sure, you can shoot the zombies all you like, and they will fall down.. it's just that they'll get back up again... On top of this, rather than being told you're great, like most FPS games, you've got an aggressive NPC on your case and in your ear, who'd like nothing better than to see you end up as a zombie's breakfast, and is going to let you know any chance he gets.
More information about the mod can be found on the project website. Look out for the other two mods being released this summer: Dear Esther (Half Life 2) and Antlion Soccer (Half Life 2 Deathmatch)