Best horror movies to see?

I'm wanting to watch some for real horror movies. None of the crap like Shaun of the Dead or Zombieland. I need PURE HORROR! Something that'll have you checking over your shoulder every few minutes to make sure your not going to be killed. I love love love horror, but I can never find a movie that's horrific enough to satisfy me. The Grudge and Nightmare on Elm Street aren't really scary to me and I've seen them 10 times by now, so any other ideas? Also I prefer movies that are made from 1990-2011 please.