Here is everything you need to build (I'm still looking for build instructions to figure out how the hell I'm gonna do it but ok) Amnesia: The Dark Descent. Included are project files for Visual Studio 2010 and CMake for Linux & macOS (that I'm ripping my hairs because they are not super clear on the build flags needed to do this).
This is probably also the most enjoyable engine to work upon, hopefully people would be able to make their own standalone games with it (then again, I've been hoping for the same for morrowind, so I don't expect too much).
We encourage everyone to contribute code to this project (believe me, I need this help), so just sign up for a github account, create a fork and hack away at the codebase (I mean, are they wrong?).
All code is under the GPL Version 3 license. Read the LICENSE file for terms of use of the license.