Event Horizon - The first time I watched this I kakked myself. You need a good sound system to fully appreciate the pant wettingness of this movie though.
Alien - Ok, I must have been 12 or 13 when I first checked this but I slept with my head under the covers for about a week because I was scared of face huggers (I even rationalised they would survive re-entry cause they are fire resistant and would use acid to burn through the roof to get me )
Mirrors was about that guy Kiefer Sutherland where he becomes a night watchman at an abandoned shopping mall if i stand corrected which turns out to be haunted and the demon that lives there cannot die unless the original chick comes in and tries to save the day...
Really scary and the ending was quite twisted but really really awesome