Descent
Beware that this movie is basically gore porn, with attractive women no less. I didn't know that when I saw it and left the theater feeling kind of sick, and that never happens to me.
Aliens. Required for ALL DFers.
That's the same as "Alien?" I actually tried to rent that last night but Blockbuster didn't have it.
Thanks for all the suggestions keep 'em coming. Also I forgot my only restriction, no damn anime, except by the dude that made Spirited Away.
Aliens is the sequel to Alien and they're both excellent.
Miyazaki (Spirited Away guy) is mostly okay but Nausicaa is definitely not for people who dislike most anime.
While we're talking anime movies, Paprika is an absolute mindfuck (in a good way) and avoids most anime cliches. If you're into cyberpunk, Ghost in the Shell is as close to a good cyberpunk movie as you'll ever see.
Die Hard was already mentioned, but it's THE action movie.
Braindead (AKA Dead Alive) invented the rom-zom-com (romantic zombie comedy) before it was popularized by Shaun of the Dead, and did a better job of it too. It's nonstop action right from the start, it's filled with hilariously over-the-top gore, it has great characters, and it was directed by Peter Jackson of LOTR fame.
Das Boot. It's U-571 without the Hollywood. Watch the full 5-hour version.
Good/Bad/Ugly was already mentioned. Clint Eastwood is also great in Where Eagles Dare.
Miller's Crossing and Once Upon a Time in America are two outstanding (and different) takes on Depression-era mob violence.
Oldboy and La Femme Nikita are both enjoyable action dramas. Oldboy in particular has a very strong plot.