All Coen brothers films are absolutely top draw:
The Big Lebowzki
O brother where art thou
Burn After Reading
No Country For Old Men
A BIG shout out to The Mist. Keever mentioned it in his OP. This is an underrated classic. One of the most horrible haunting endings ever.
Other than that, you guys have mentioned some great films.
... and, wheres the Tarintino love? The dude is a genius who makes better and better films. Inglorious Basterds was absolutely magnificent.
Beyond that, anybody seen Moon? Fabulous film.
Im a massive fan of horror. Generally, English speaking countries dont do it very well. The koreans absolutely rock.
We Are Who We Are
I could go on... but I'll stop now.
Moon is really, really good.
Under rated film I just thought of; The Negotiator with Samual l Jackson and Kevin Spacey.