What movies will still give you nightmares?

I'm a pretty big fan of horror films guts blood and all, and love it even more so when the characters are demented ex: "High Tension" and "It". Though I am never really bothered by them. Probably because no matter how real the situation in the movie seems, my naive little head can't picture these things getting played out in real life. ( though there are exceptions, ex: women stuck to toilet for two years) Even now the only movie i can remember giving me nightmares and disturbing my sleep is the result of watching "Arachnophobia"  I am so confused by this, because compared to all the other movies I've ever watched this movie is tame, and really is the most unrealistic. Not to mention I'm not even that scared of spiders so what the hell? (centipedes are another story :P)

So my question to you all is; what movie(s) gave you the creeps, use to/still do. Would you watch it again, and if you did do you still have the same reaction?