I don't know if this is still in theatres, but it's definitely worth mentioning. Cowritten by Avenger's director Joss Whedon, this is - without a doubt - one of the biggest OMFG horror movies of all time.
It starts off simple enough, a bit of a play on the Sam Raimi Evil Dead kind of movie. Cabin in the Woods, basically the most common horror movie trope there is. But then ... well, the end is just a complete mind f***. Absolutely amazing to watch, you can't keep your eyes off it, but completely and utterly thrilling. The first part of the movie isn't as scary as a horror movie could be, but that's just leading up to this kick ass ending that just gets you all riled up and excited.
And there are some of the funniest lines I've ever heard in a horror movie. It's a great flick. Can't recommend it enough to anybody who loves horror.