The Changeling
This movie came out in 1980. It didn't have an all-star cast, nor did it have a particularly earth-shattering advertising budget. What it did have, was genuine production and a terrifying plot. The Changeling is about a composer who moves into a haunted mansion in Seattle, but in this case, "haunted" means something that you may have never seen before.
The Blair Witch Project
The Blair Witch Project was the first movie to really nail the found-footage horror genre. A group of local college kids treks into the forest to inspect the mythical witch that lives within the trees. Without this movie, there would be no Paranormal Activity, Insidious, or Devil Inside.