Time for you to name your ultimate scare in a video game or film, or both.
Mine (very sad and probably not that scary by todays standards).
Game : Resident Evil (the first one) : Where the dogs burst through the side window towards the start of the game.
Film : The Descent. When you see the monster through the night vision of the camcorder the girls are using. I slopped half a curry over my lap when that happened.
Sorry for the typo on the subject line!

Nocturne.
Never jumped during a game until this one. Walking through a vampire's castle when your character walks past a mirror where the reflection doesn't look quite right, when you go back to the mirror a bad guy jumps out with appropriate loud noises. Got me good.
Edit: Throw in F.E.A.R. as well, the bit where you climb down a ladder and as the usual cinematic unfolds you catch a glimpse of Alma standing at the top glaring at you. Damn creepy little girl.

The hospital in Silent Hill with the random banging noises.
Or maybe even parts of Project Zero.
I love horror games :)).

Half-life
Undying
Clock Tower

Call of Cthulhu dark corners of the earth
The bit when your being chased through the hotel having to lock each door behind you only just locking each one in time gave me a super sweat and panic.

Thief: Dark Project. The Haunted Cathedral.
I had wandered into a fairly open area. I had carefully watched and listened for nasty things, and finding nothing, I set about to pillaging. I paused mid-lockpick to check again, and discovered that a zombie, without so much as a mumble, and with the grace of a ballerina, had silently tip-toed up behind me.
Stalker: Shadow of Chernobyl. Military Warehouses.
There's a certain phenomenon, where your vision goes blue and blurry, and you hear this kind of spooky moaning noise. Experienced stalkers know what this is, and how to deal with it. I was not experienced. Having no idea what was going on, and not seeing any possible cause for the weirdness, I panicked and ran. Screaming. Like a little girl. Through the bloodsucker infested village.

I knew someone whom I told that to, but when they played it, jumped down every ladder to make sure they never saw it. TBH thats a bit weak if you ask me.
*Spoiler Warning*
My best (worst?) scare has to be in Condemned: Criminal Origins. Its when you're exploring the school and you open a locker to find the body of a guy who has been horribly mutilated. To get a positive ID, you have to take a picture of his face. The very second you get the zoom into focus on his, he bursts into life begging for help. It wouldn't be so bad, if he didn't look like one of the aliens out of "They Live!"
I can usually take a lot, but I had to smack the Esc key and give myself a minute to calm down. I've never had to do that before in a game.

The first parts of System Shock 2 scared me pretty good, the Many are just a fantastically creepy enemy. Also Fatal Frame really managed to creep me out, I remember a part late in the game where you had to follow a blood trail throughout the mansion, it was creepy simply because it stood in stark contrast to the rest of the completely bloodless game.
Call of Cthulhu: Dark Corners of the Earth was fantastic for the first three hours of the game, then they decided to give you guns and in a flash the atmosphere and uneasiness the game had built up disappeared to make way for shooter elements. Still I enjoyed the game and the hotel chase is one of my favorite moments in gaming.
Also Clock Tower was nice in that it had the jump out of your seat scares, but also a very tense feeling throughout the rest of the game because you never quite knew where that scissor wielding bastard was hiding.
And of course I cant leave without mentioning Silent Hill. I could go on for days about all the things I love about those games, from the slow buildup of tension to the screwed up logic of light bulbs in a can, to the stories so packed hidden meanings and symbolism, but I don't want to do that. So enjoy these videos instead.
***WARNING SPOILERS***"I'm the same as them": not really a scary video but it is one of my favorite moments in the series. Dark, bloody, sad, and beautiful.
Main theme on piano: Just take a listen, I have nothing more to say.
That being said though most horror games don't really scare me. Most games and movies these days seem to equate horror with either giant "BOO" scares or just covering the screen in blood and it just comes off as trying way to hard.

I seriously need to check out Call of Cthulhu....

Film: Probably the best stayer is a small moment from Aliens when they're looking at the facehugger in a tube and it jumps onto the glass. Its a tiny thing but it still makes me jump even after having seen the film more than 100 times
Game: Ocean House Hotel in Vampire Bloodlines, the first time I went though that it creeped me out, great stuff. Runner up would either be the fear moment mentioned earlier or maybe the destroyed village in Velvet Assassin though the latter is more atmospheric fear rather than "Boo! Nazi!". Condecmned is definitely creepy and I'd imagine it topping the list except its so boring to actually PLAY, especially with the shitty fighting mechanic. Might have to youtube the whole game
Real Life: Bam!

Game: No real scares:
The penultimate(?) level in system shock 2 creeped me out. Walking through this big organic mass with grinding teeth and pockets of the ship that was not taken over. Resources and places to hide are pretty much absent.
Aside from that, it's those jumps you get, even when you know it is going to happen. doo-dee-doo walking down a corridor, ooh, a big metal door? Hmmm, let's open it and OH GODS! a shambler!
Film: watching ringu-2, alone in the dark at night, when it's very cold. No jumps or screams, just that eerie disturbing feel even though I know better and it's just a movie.
But as a little kid, Neverending Story, because of G'mork.
Though I do know a few people I can quickly get out of a room, if I walk up to a mirror and chant 'candy-man'

I've had some fair scripted scares from games, but the best frights I can think of come from Return to the Cathedral in Thief 1. Undead in Thief are seriously creepy, and this place was crawling with them. It's all unscripted; it's scary because the creatures are scary, and dangerous.
An honorable mention goes to the ghost in the sitting room in the first Alone in the Dark. It just sits there quietly, not moving, staring at itself in the mirror. It won't bother you... unless you touch it. Please, please don't touch it.
That game had a lot of bizarre moments and eerie denizens, but this was my favorite.

This. And moments with little girl at the first 1/3 of Call of Cthulhu. You know which ones I mean :D I was litelarry trembling, mumblings of your hero among the lines 'I want to die' didn't really help either :D

Resident Evil 1. Probably because I was really young, but that shit was scray.

I can only think of one game that I actually found scary, and that was the first time I played it.
The CD version of [url=http://en.wikipedia.org/wiki/Alone_in_the_Dark_(video_game)]Alone in the Dark[/url].
The mix of the creepy audio (including voiceovers for the books and letter you read) and the lovecraft style horror was very creepy for me!
I get creeped out by films all the time though, depending on what the horror is.
I have the Ring/Ring 2 box set, and in the extras of one of them (the first I think) theres an extra which plays what looks like the tape from the film, but different. I didn't find out how it finished though!