The REC movie site is one of the most immersive Flash video interactive pieces I've seen in a long time. The site puts you into the movie as though you are watching video captured from an observer who is actually in the scene. There's a qality to the video and the storytelling here that feels very reminiscent of the Blair Witch Project, and here it is an equally successful vehicle.
After exploring and experiencing some of the terror for a few minutes, check out the video footage of audience reactions at a screening of the film.