The source code for
DOOM III has been released and you can find it on
GitHub. Now, there aren't any assets to go with it, but you can poke through and see how the game was made to run. If you're a developer, there's a lot of really good information here and you can take the code to do with what you will.
If I had the time, I'd pour over it to see how things get done and learn a bit. I wish more game developers released their code though as this is a great way for up and coming developers to see the inner workings of a published game.
DOOM III was released in August 3, 2004 so it might be another seven years before we see the release of the
Rage source code.
