Link(Movie details): http://www.imdb.com/title/tt1038988/
This is a spanish little gem in the "first person documentary horror" genre, same as the first Blair Witch movie (which I have not seen by the way). While pretty famous in Spain, I'm not sure how known it is worldwide, I think you will at least know the american remake "Quarantine". I like the original better, just my opinion.
The movie is shot from the point of view of a cameraman in a fictional TV show called "Mientras usted duerme" (While you sleep), as he films reporter Ángela Vidal during a usual Barcelona Fire Dept. work night, until there's a seemingly rutinary emergency call from an old apartment block. I don't want to give away anything further than this, so if you want to know more on the plot, you are adviced to watch the movie instead of checking the internet for details :).
The actors do a nice job, the set is claustrophobic enough, it actually gets to build up quite an atmosphere, and apart from some a bit lamish moments, the movie delivers in my opinion. Directors Jaume Balagueró and Paco Plaza also have some more interesting work, which I might advice sometime in the future.
Now that the sequel is about to hit the spanish streets (well, like in some hours actually :P), I recommend you watch this one, and if you already did, just feel free to discuss, but be careful with spoiling stuff.