Obvious spoilers and I don't feel like tagging them all.
Wow, no mention of the blood test scene from The Thing yet. For shame.
The Demon appearing in the mirror in Jacob's Ladder
The ghost appearing in the school, saying "Why are you sitting in my chair?", and jumping at the lead actress in The Eye (03)
Both dream sequences during An American Werewolf In London feature great jump scares. As does the death of the couple.
The alien appearing in the air vents when Dallas is investigating them during Alien.
The hobo thing behind the dumpster in Muholland Drive
The body in the bag moving in Audition
The girl in the closet in The Ring