Stephen King's The Mist

This was a good, incredibly intense horror movie. It centers not only on horrific monsters shrouded in fog terrorizing a group of people, but also how people themselves can become monsters when placed in an impossible situation.
Set in a small town in Maine (like most of Stephen King's stories) The Mist follows the aftermath of a violent summer storm, during which the art studio of protagonist David Drayton (played by Thomas Jane) is crushed by a dead tree belonging to his neighbor Brent Norton (played by Andre Braugher). David and his son Billy (played by Nathan Gamble) then go to the local supermarket to pick up some supplies, where they find the power out and a sizable amount of the town's population there with the same idea as them. All at once, a thick white mist rushes in off the lake, and one of the townspeople, his face covered in blood, runs into the supermarket from the street, screaming about something in the mist. The plot takes a frightening turn from here all the way until the the end, hardly letting up for more than a minute or two at a time. The mentally unstable religious zealot Mrs. Carmody (played by Marcia Gay Harden) is an amazing antagonist, and in many ways she is almost scarier than the bizarre monsters that come out of the mist.
If you like to be scared, you should see this movie. A word of caution, though: the ending is almost gratuitously horrible, and doesn't even match the ending of Stephen King's original story. I suggest you follow the advice of one of the user reviewers on Rotten Tomatoes and just walk out the minute the people in their car run out of gas. I don't want to give away any details in case you do want to hang in there until the end, but I know that you will be shocked and disgusted, and not in a good way.

Check out some reviews at Rotten Tomatoes and Metacritic.