Paranormal Activity 4 and a handful of short films I've seen recently fall into this category too but The Den has done it best as nearly all of the deathly horror of the film takes place on the protagonist's computer screen. Elizabeth, the film's hot protagonist (and true to Horror form we see her in her underwear for no reason at least once) is writing a thesis on internet chatting culture specifically on the site"The Den," sort of like Chatroulett's fake bastard cousin. Yeah, this is far fetched, but whatever. As she begins her "study" she comes across something that might be a murder and then her whole real world begins to collapse in on itself as it merges with that of an alternate underbelly of internet existence brought on by a torturing hacker, or is it hackers?
Blair Witch, Paranormal Activity) and directors for whom this gory genre acts as a portfolio/springboard (Raimi, Peter Jackson), The Den presents a new vision of the state of film and technology. I am not telling you the ending of the movie, this is one that I recommend you watch on your computer. With the light off. Or on. Full disclosure: I turned the lights on.