[Update #2]: Amnesia: Rebirth has now been unleashed for PC and PS4, and unsurprisingly, it's pretty much the original game with new content and a few extra bells and whistles.
[Update]: Amnesia: Rebirth's lengthy preview offers plenty of great atmosphere and a glimpse at the monster.
If you ever wanted to see what kind of dark sorcery happens behind the curtains of game development, today's your lucky day. With Amnesia: Rebirth launching this October 20th, Frictional Games have decided to release the full source code for Amnesia: The Dark Descent and A Machine For Pigs under the GPL v3 license!
Much like how the warning on the website will tell you, this doesn't mean that Amnesia is now suddenly free. Frictional still owns all of it, but this does allow you to peek under the hood to see how the engine works, how the various gameplay mechanics were handled, and perhaps even puzzle over some particularly arcane piece of code. After all, Amnesia is over 10 years old at this point, so odds are there's some janky nonsense in there - there always is!
Even if you don't particularly care about the technical aspects, this is still a major victory for the modding scene as the source code can potentially allow them to create just about anything within the Amnesia framework. So while the odds are slim given the game's age, don't be surprised if some rather intriguing mods start popping up in a few months.
Have fun poking around in the code, and if you need a little bit more information about this whole release, you should head on over to the Frictional website.