The Descent really scared the crap out of me. Anyone who saw it will understand this - I ALWAYS check around my car when I get in it.
Mirrors scared the crap out of me, cuz of the way it ended. After that movie, I can't stand mirrors in the dark.
Chucky scared me as a kid, and am still afraid of weird looking dolls to this day, and probly always will be. I remember when I was younger, I had a doll that I wrapped in a plastic bag and threw it in the corner of the closet, piling clothes on it. Scared. To. Death. And I used to constantly check behind the shower curtain, but have grown out of that... Mostly.
I'm a big baby when it comes to horror movies, so I'm sure I'll think of more. Those are the big ones that really had an impact on me though.