Wizardry 4 feels like a horror game at times. Trebor's Ghost, in particular, adds a sizable horror element to the atmosphere, in addition to having to worry about all the do-gooders who want you dead (and many of them *are* powerful enough to kill you outright).

Note that you should really play Wizardry 1 first, or else you will reach a puzzle in Wizardry 4 you won't be able to solve.

Scratches: Director's Cut

Call of Cthulhu: Dark Corners of the Earth

The Suffering: Prison is Hell

I'd also recommend reading the story. It's only like four or five pages long.

The Lost Crown, one of the best adventures I've ever played. It's so damn creepy and has a very intense atmosphere.

Unfortunately it's not available here, so please vote for it:
https://www.gog.com/wishlist/games/the_lost_crown_a_ghosthunting_adventure

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You might enjoy Penumbra, the trilogy's currently on sale for $2. It's by no means an A+ horror game, being indie, but it might be your style (or you might find it boring). The 3rd game was very lame but the first two were decent. I enjoyed the ending of the first one especially.

As with all games, watch some LPs of ones you're interested in.

Dead Space while definitely more monster packed, the spaces in between fights, had a great atmosphere. I especially like searching the bathrooms because of the blind corners and tight spaces.

- Alone in the Dark
- Realms of the Haunting
- The Legacy : Realm of Terror
- Call of Cthulhu : Dark Corners of the Earth

Pretty much anything with a Lovecraft influence is going to be big on atmosphere and lighter on action.

And I have to add these two gems (first-person p&c adventure games):
- Darkness Within - In Pursuit of Loath Nolder
- Darkness Within 2 - Dark Lineage

Of course, whether or not one finds a game creepy/terrifying differs from person to person, but I found these games, especially the first one, to be extremely creepy. Besides, that atmosphere, that music (the first one), you have to play them.
Never have I played a game (and I have played almost all Lovecraftian games) that nailed the atmosphere of Lovecraft's tales (as I perceived it) as well as those did.

EDIT: The second game finishes the story the first one started, so play them in order.

I'm not a big fan of horror games myself but I had great time playing Gabriel Knight 2.

I'd say Dark Fall as well. The puzzles can be really difficult but the atmosphere in all three games is fantastic.

Dear Esther was very creepy to me, being fully immersed in it. Same with Pathologic.

FEAR has a fantastic atmosphere, in-between the action.

It's not on GOG but Alien Isolation was very tense and stressful. Call of Cthulhu: Dark Corners of the Earth had a really creepy mood as well.

May I recommend the John DeFoe quadrilogy?

5 Days a Stranger
7 Days a Skeptic
Trilby's Notes
6 Days a Sacrifice

They are all very atmospheric and creepy. And they are all freeware.

Bump.

Clive Barker's The Undying springs to mind right away.

Year Walk
The Vanishing of Ethan Carter
The Moon Sliver
The White Chamber (free)
Serena (free, but available only on Steam, I think)