Pretty much anything from any Silent Hill game is a good source for horror inspiration. They always nail suspenseful, moody environments.
Something that looks scary and pop out of corners, eg. mutants, are good for instant fright, but they don't last. Once you've seen one, their "scariness" begins to decrease exponentially with each encounter, so use "jumpers" in EXTREME moderation.
I've found that the best way to utilize a "jumper" is to build up a lot of suspense over it a few times without it actually happening. Then surprise the player by throwing something at them when they think they're safe. After that, they will never know when to expect anything and will get up tight over small things.
And just for the hell of it, you could check out my "Asylum" map for an old mini compo.
http://twhl.co.za/mapvault_map.php?id=4564