Triangle is an interesting recent film. Although some of the blurb makes it look like another crappy slasher, it's actually much more interesting and unsettling than that, and I think very well structured.
That movie was totally unfair. So full of holes and absurdity, yet I loved it to death anyway. To be fair, you can't really do a silly premise like that and make it airtight. That Christopher Smith has some style, though. Black Plague was good too, if you like him as a director. But I second Triangle - it didn't seem to get much marketing, but despite its many flaws, it's pretty memorable. It's got one of those good, minimalist horror soundtracks that sticks with you, and you never fail to associate the music with the movie.
As for recommending fine horror movies, I'll go with the generic greats:
Alien, The Thing, The Exorcist, the first two old-timey Romero zombie ones, Halloween, etc.
If you want to get more recent, the standard has to go down, but off the top of my head, there's Pandorum, 1408, and The Mist at the very least.
The horror genre's been thin lately - though not for lack of trying, with The Thing remake and some pseudo-Alien stuff from Ridley on the way. People got hype for The Descent, but it's just a remake of a better film, though that director's previous movie, Dog Soldiers, is a little bit slept on. People got hype for paranormal activity, but that found footage stuff only works for certain types of people. We had quite a run of exorcism-themed movies there, but none of them were particularly good. There was a sci-fi found footage movie that nobody liked. It's just not a great time to be looking for quality horror at the theater.
. Amusing experience, even with friends around. 
