Besides the usual favorites, here are some (I think) under-appreciated ones.
Friday the 13th IV - I'm partial to this because it's the first real horror movie I ever saw, and I couldn't close my eyes in the shower when washing my hair for months, but it's also got Crispin Glover.
The Host - Korean film, I think you can stream it on Netflix, really well done monster movie.
Let The Right One In - watch the original if you can, not the american remake, although I had heard the remake's not bad.
Alien Resurrection - I know most people hate the 3rd and 4th installments in the series, but I like them. I especially like the comic book kind of feel to this, and Ron Perlman is great.
Zombieland - more funny than scary, but it's a great movie to watch this time of year.
Tusk - Kevin Smith made a good movie again. Made me squirm and laugh.
The Thing - John Carpenter version - probably not under-appreciated, but I didn't see anyone else mention it.