The Blair Witch Project..didn't find it remotely scary when I watched it the first time, thought it was a waste of time and didn't really see what all of the fuss was about, however I caught it one time on really late so decided to put it on whilst on the pc, and just couldn't help constantly glancing at the screen and totally getting into the story..found the ending really creepy too. It's definitely a film that neeRAB two watchings before really saying it's rubbish.
EDIT: *jaw dropped* is the poster who has posted before me really who I think it is!?!?