Silence of the Lambs
came out in 1991...it's pushing 2 decades.
I'd say The Sixth Sense
is one of the more important horror movies of the last decade...it ushered in the twist ending and the PG13 horror movie.
I guess I would also include Saw
on that list as it launched a seemingly unending stream of sequels...probably the first "big" horror franchise since A Nightmare on Elm Street
Edited to add: I would consider these to be important horror movies, although I don't consider either to be particularly good...certainly not great.