I have a few, and I will try to not mention TOO MANY that have already been mentioned, but some will be repeats from previous posters...
The Mist: My second favorite Stephen King film, behind only The Shawshank Redemption. I feel like everyone who sees it thinks it is great, but most people haven't seen it. That ending is just awesome, too.
Frequency: I thought my old roommate was BSing me when he told me this movie I had barely even heard of was one of his favorites. Such a good movie.
Drag Me To Hell: To me, this is one of the best, most fun, horror movies of the last decade, at least. I love it, but it slipped under most people's radar (the internet movie nerds like us don't count; believe it or not, the average person probably doesn't even know Sam Raimi directed Spider-Man, much less Evil Dead).
Trick 'r Treat: Basically the same as Drag Me To Hell, except this one didn't even get a theatrical release.
Titanic: Yes, it is the second biggest movie of all time. However, it gets trashed all the time. When it came out, all guys thought it was "gay" and hated it because all the girls loved it and wouldn't shut up about it. Now, after that time has passed, it still gets hated on by a lot of cinema elitists because hating James Cameron is the cool thing to do. I watched this movie for the first time in almost 15 years when my wife dragged me to see the 3d rerelease. This is actually a damn good movie.