So: Let me start you off.
1) People standing behind you. (See above.) I hate sitting at my desk with my back to the door. Dammit.
2) People standing behind you, seen in mirrors or reflections. (I don't know if this was in The Ring, but I am always afraid I will see Samara in my bathroom mirror. Dark TVs started weirding me out for the same reason.) Also included: shit you think you see out of the corner of your eye.
3) Something behind the shower door/bathtub curtain. You know nothing is there, man. But you hate it when your little sister makes you check for her. Hell, just showers and/or bathtubs in general (Psycho, Diabolique [no, man, the good version, the original French version], The Grudge, the old lady who had dissolved into mush at that freaky guy's house at the end of Silence of the Lambs--I bet half of you didn't even notice that).
4) Something behind a door. Like, "THE DOOR IS OPEN." ACK. (And yes, I will stop posting that link... as soon as I get over it. Eventually.) There are actually a lot of stories about doors that only the narrator can see, and the narrator's ensuing frustration that it is creeping his shit out but everyone else thinks he's crazy, and inevitably he goes nuts in his attempts to either open it or make people see it. As soon as I can remember a specific story, I'll link to it. Unless you pay me not to.
5) Closets. Closets are baaaaad news, man. Saw and The Grudge come to mind without my ever having seen the actual movies.
6) Deep or murky water. You know, just opaque enough that you can't tell what's in there. Bonus points if it's in a bathtub. Super mega bonus points if you're in it.
7) Masks. Particularly weird animal masks (Saw, Donnie Darko, tons more that I don't care to think about right now. And isn't there a really freaky little throwaway glimpse of a guy in a fur suit kneeling and a man in a tux sitting on a bed in The Shining?). I think the scariest possibility with a mask like that isn't what's under the mask--it's that it might not be a mask.
8) I don't even have a name for this, but it was in In the Mouth of Madness. Sam Neill is walking past the counter of the little hotel where he's staying, and the camera is looking at him from behind the counter, and what the old woman at the counter is trying to hide from him is that she's got her wizened old husband cowering down on the floor, chained up and naked. And Sam Neill just walks past and doesn't see. The whole thing lasts like two seconds and is never mentioned again, which is what makes it so freaky. It reminded me a lot of that episode of Ren & Stimpy where they're selling rubber nipples door-to-door and that weird horse answers and he's hiding, like, a walrus in a gimp suit or something behind him (my memory is a little foggy here), and the walrus whispers, "Caaaaall the poleeeeece." End of scene. That's what I'm talking about here--that weird little "What the fuck was that?" moment you get every now and then that is totally ignored by the rest of the movie/book/show/whatever. Because, seriously--the fuck?
9) People standing with their backs to you. I didn't really think about how utterly freaky this is until I saw the Jack-centric episode of Lost and his father kept standing with his back to the camera. Just hands down at his sides, completely robotic. Because you keep wondering what's wrong with the front of him, you know? It's also kind of zombie-like. The end of The Blair Witch Project tapped into this in a major way.
10) Clowns. Of course. I'm not scared of "normal" clowns, although I know people who just don't truck with no clowns no way. There's just something about a particularly evil-looking clown that just doesn't look... human.
11) The dark. I mean, of course we're scared of the dark because we can't see what's going on. But I think there's something at least a touch irrational about the things we start imagining might be there. Like that whole "Aren't you glad you didn't turn on the light?" urban legend.
12) Eyes. This is a whole subcategory right here. Shit being done to people's eyes. Shit being wrong with people's eyes. Scary devil eyes or Anakin goes to the Dark Side eyes or Event Horizons eyes. *shudder* There's an entire ten-minute section of Minority Report that makes me curl into a little ball and sob because I have a severe horror of Eye Stuff.
13) I'm not doing a #13, man. Oh hell no.
Your turn! Meanwhile, I will be sleeping with my Galadriel doll tonight. If I can sleep.