Dark Seed - Point and click psychological horror adventure game
Call of Cthulhu - First person thriller-horror adventure with shooter and stealth elements
Gabriel Knight 1- very unsettling, atmospheric adventure game
Clock tower - Haven't played this one yet.. It's a point and click survival horror game
Dracula: Origin - Not exactly scary, but it has a terrific atmosphere and gripping story

Hope that helped you.

Still Life is the adventure game that creeped me out the most. I loved that game and just bought the second one. Can't wait to play it. Haven't played most of the games people here are recommending though, unfortunately. I will say though, some of the scariest games I've played are Silent Hill 2 and Condemned: Criminal Origins.

The only time I've been truly scared playing a game was when I played Vampire: The Masquerade - Bloodlines, the Ocean House level. I couldn't wait to get out of that house

Agreed.

Shudder to think of it now.

Clock Tower id a pretty good recommendation, that was such a weird yet compelling little horror story. Despite being surprised a few times boy that guy with scissors, I couldn't help but laugh at him more than once, such a bizarre character...

Oh and Silent Hill 2, despite not being an adventure game, is one of the most genuinely dramatically horrific and disturbing games I've played that really portrays human behaviour in their characters realistically. One of my favourite games!

Been alluded to, but the creepiest adventure games I've played have been the
Chzo mythos ones, made by Yahtzee, the guy who does the Zero Punctuation reviews. Creepy, fun and best of all free.

I only ever played the first title of the series, but I didn't find it particularly creepy. I think it was the characters and their humour which just took the edge of that off the game for me. Don't get me wrong - I loved it, I think the characters and the dialogue were great, just that it wasn't necessarily creepy...

To be fair, both the story and atmosphere develop over the series. Chzo (who doesn't appear in the first game) himself is definitely creepy, and one game in particular (pretty sure it's Trilby's Notes) has some rather more graphically violent stuff, and would screw with you in certain areas by occasionally transitioning the screen, flashing up hellish images accompanied by freaky whispery incantation-type noises.

I see, I see... does sound particularly good. I've been meaning to follow up with the rest of the series, so now may be as just a good a time as any to do so!

I first played the 7th Guest with my dad when I was 8. I've played it every few years since then (I'm now 25), and each time I play it there is always a strange nervousness that comes over me. I turn out the lights and crank the speakers. Still gives me the shivers wandering through that old mansion. Not so much the fright, but the crazy awesome atmosphere. The graphics, in my opinion, hold up surprisingly well today. I'd like to see an HD remake.

I think the 11th Hour lost something in the translation - it had a nice feeling to it, but doesn't have the same effect as it's predecessor. I was about 12 by the time I played the 11th Hour. I'm glad I got the Win 98 version of 7th Guest so I can still play that. I wish I could get the 11th Hour to work again. If only I could figure out how to work DosBox

I'd have to say The Lost Crown has the best sustained overall atmosphere, although it only has a few jump moments. Scratches had the best jump moments for me, and Sanitarium made me feel a little sick all the time because of the ideas and imagery, but wasn't very scary.

I've only played it for a couple of hours, but Amnesia: The Dark Descent is becoming a strong contender to the title.

It's like a mash-up of Call of Cthulhu: Dark Corners of the Earth and The Cradle from Thief: Deadly Shadows. Could very well turn out to be the best Lovecraftian game I ever played*. Very Charles Dexter Ward'ish so far, and despite the lack of NPCs to interact with, there's some lovely voice work - One of the actors sounds uncannily like Orson Welles.

*(Though Shadow of the Comet still holds a special place in my heart as the first cd-rom game I ever bought. Nor will I forget Prisoner of Ice, which I wrote a walkthrough for that became my first piece of writing published in a printed magazine).

Yay! I'm glad to see that I'm not alone in thinking that Amnesia reeks of Dexter Ward. I was constantly thinking of that story while playing. Great stuff.

Cast another vote for The Dark Eye and its disturbing sibling Bad Day On The Midway.

Scratches
Darkness Within 1

Lost Crown - had a spooky environment. Gave few scares. But I think it leaned more towards thrills then chills. Great game nonetheless.

Black Mirror series in no way is horror. Just a little spooky touch in the end of the 1st part.

Hmm... Scary games huh?

Definitely Clocktower 3.

Spoiler:

One time my friend decided to play it in complete darkness. She was in chapter 4. Creepy hospital. Hid in the bathroom stalls. He seemingly disappeared, the music went away and we heard him teleport. She comes out. Starts walking. "Hello Alissa!" We nearly crapped ourselves because he popped out of the broken stall!

Spoiler:

Also one time in chapter one I was playing and I was expecting the guy to pop out of a door. The other people didn't. Everyone, including me, jumped when he did because I was right in front of the freakin door!

Haunting Ground, similar to CT3, but more realistic.
If you happen to like shooters there is Dead Space, F.E.A.R., and Bioshock.
I've also heard Fatal Frame is creepy.
Whoever said Siren, that stuff is way scary because you really have no idea what you're doing.
Exmortis is a very creepy freeware series.
Stone of Anamara is also kinda creepy.
Hope that helps! :)

Exmortis

The original
The sequel

Also throwing my vote in for Scratches.

The only adventure games I remember scaring me are those with real-time action elements and the possibility to die - most notably the original Alone in The Dark trilogy and Shadow of The Comet.

But the best horror game I played is probably The Dark Eye.

I haven't played Scratches yet.

Scariest online game ever!!

Scratches.