"The Descent."

The Descent is the best horror movie I've seen in years. It's the type of movie that's classified as horror but is really just a fantastic movie that happens to be scary, in the same vein as the Alien movies and The Shining. Topically, the film is about a group of girls that goes spelunking, and things start going wrong. What makes the movie so good is that it focuses on relationships, tragedy, and acceptance (or lack thereof). I'll refrain from further discussion since much of the impact of this film lies in your interpretation of what happens.

One final note - get the unrated director's cut. The principal difference is that it contains the original movie ending. Apparently the ending released in theaters in the U.S. was slightly edited (I obviously didn't see this in theaters), which is a shame because I can't imagine the movie ending any way but the original way the director intended.