If you played the first two Amnesia games and wondered something a bit more exhaustive to fill in the gaps between the two games, a group of fans have come to the rescue.
It's called The Streets of London, and it's a fan-made "midquel" that begins on the same night as proceedings in A Machine for Pigs. It's been in development for a couple of years and adds an environment "with the sprawl and expanse of The Dark Descent" within the London setting of the sequel.
According to the ModDB page, The Streets of London is more driven by the story and monster encounters than puzzles. You can download the 2.35gb mod now, with installation as simple as dumping the London_files folder into the root Amnesia directory (The Dark Descent, not A Machine of Pigs). There's also two of "unavoidable" crashes in the current release, although the creators say the game saves automatically beforehand so you should be able to reload without any issues.
Update: Added a small note about where to install the mod files. Make sure it's in the right place!