Last Post: 3:06:29am, 11/16/2017
I'll second Cube and 28 Days Later. Cube 2 and Cube Zero are more of the same, if you dug the concept.
A bunch of these aren't strictly horror, but are movies I enjoyed that have elements of horror.
The Descent
Green Room
Housebound (Horror/Comedy)
Krampus (Actually kind of tame, but it's a fun Christmas-themed horror movie)
Buried (2010, more of a psychological thriller, but the concept is unsettling)
Sunshine (Sci-fi with horror elements)
American Psycho
The Gift (2015, more of a suspense movie, but kind of creepy)
The Babadook (Although I found the kid to be ANNOYING, I thought Essie Davis was great)
Dark City (kind of a stretch, but it has a great gloomy atmosphere)
Donnie Darko
Battle Royale
We Need To Talk About Kevin (Even though I don't like Tilda Swinton that much, the performances are pretty unsettling)
Let Me In (2010)
The Machinist (I need to rewatch this, actually)