Shalebridge Cradle of Thief 3 is easily the scariest "horror level" out of any non horror game to date. I wasn't even wearing headphones while playing the level and I damn near had a heart attack when one of the enemies surprised me from behind while I was lockpicking a door. This level alone was scarier to me than Dead Space and many other horror games are from start to finish.
In general all of the Thief-series games have horror elements and scary atmosphere. The fact that they're first person sneaking games where you generally have to avoid detection and that they all have really good audio design makes them so much more intense.
Return to the Cathedral is another scary level from the Thief-series.
System Shock 2 and Vampire: The Masquerade - Bloodlines are both FPSRPG games with a horror theme. VTMB has some scary and gory missions. SS2 is a proper survival horror game though, and one of the scariest in it's genre.
S.T.A.L.K.E.R-games can have incredibly tense atmosphere at times.
This one was OK tooJ T wrote:Ravenholm.