Donnie Darko was a great movie. I plan to get it on DVD. Pretty decent website too. But I never thought of it as a horror movie.
You're right, there is no visual horror, it's just that the very concept of the movie was terribly disturbing. I was totally bluffed by the way it ended, and I've been in love with Jules' "Mad World" song ever since.
In true horror genre, I'd say "The Thing" (the 1984 version). I really liked "Invasion of the Body snatchers", but like most 1970s-vintage movies it has not matured very well IMHO.