Some horror films will do anything to scare an audience. Not just the bad ones, either.
All is fair in love and genre cinema, and even many great horror filmmakers like Scream's Wes Craven or Halloween's John Carpenter aren't above using dirty tricks like jump scares and "he's behind you" moments to spook their viewers.
One of horror's longest-standing cheap tricks, however, is the undeniably effective gambit of breaking the fourth wall. Nothing is scarier (and occasionally funnier) than when a film acknowledges that it's just a movie, only for its monster to then threaten the viewers themselves instead of the movie's characters.
It can be as as small and comparatively subtle as an evil figure turning to stare right into the camera lens, or as dramatic and ambitious as an entirely meta movie which knows it's a movie, but there's one thing which the ten films listed here have in common. They are all horror films which turned on you, the viewer, (and their camera man) to scare audiences senseless.
Also, trying to keep the found footage cheating to a minimum here, so there's only one film on this list where the director opted to use that cop-out sort-of-meta framework.