I realy love The Blair Witch Project, it's true that once you've seen it once it totally loses it's scare value, but when I first watched it I was so drawn into eerything that was happening and the way the characters were afraid of what we couldn't see was fantastic. Ultimately it was an original film with an even more original technique of filming. I understand why many people would find it boring but you really do have to watch it under the right circumstances.
One of the things I really loved about the Blair Witch Project was the way that the film got a fan base way before it hit cinemas and claimed to be real footage etc. So many people were fooled which only accentuated the film's effect on people.
Anyway just thought I'd say thanks for more suggestions, many of you have listed some classic films (Alien, TBWP, The Thing etc) which would make good halloween viewing! Cheers!!