Amnesia: The Dark Descent is an indie horror game made by Frictional Games. But don't let the "indie" label fool you, Amnesia is full of quality. They released an SDK some time ago, and people have been taking advantage of this. As a result, hundreds of custom campaigns (called custom stories) have appeared on sites like ModDB.
I already have a thread on my picks for Amnesia custom stories, located here, in which you'll find more than enough info and images. So I'll be brief here.
Disponentia by Mackiiboy. One of the most original and interesting custom stories, based on a true story and a real haunted mansion. Very innovative, I consider it to be the scariest Amnesia mod.
Emma's Story by Evil Armor. It even recreates the main hall and dining room of Resident Evil.
Amnesia Hill: Harold's Story by VengeanceFilms. It's a custom story inspired by Silent Hill, but more directly than all the others. Complete with Silent Hill references, similar atmosphere, and some sound effects. It's a very well made mod and very long, there's essentially two long campaigns for it.
White Night by Tanshaydar. The most famous Amnesia full conversion mod, very well received and very original.
Through The Portal by DamnNoHTML. The most wild, mesmerizing Amnesia mod. Amazing otherworldly experience.
Mental Memorial by DnALANGE. A full conversion mod with distinct atmosphere, new monsters, new gameplay mechanics, new sounds, and is just very well put together.
La Caza by Team Ninja-Samurai. An award-winning, highly sought after mod.
Abduction by Anxt. The most true to the original game.
The Attic by Everlost. A three chapter series, ranks #2 as scariest Amnesia mod.
Out of Time by Custommapper. One of the most innovative mods, full of potential. It could use some improvements in level design/bug removal though.
The Things in the Night by DoomGod Collab Productions. One of the longest and most impressive custom stories. Unpredictable and an overall amazing experience.
There are plenty of other enjoyable campaigns, but these are the ones that stand out the most. Moving on...