An excellent movie, and I considered including it, but I think it's more distinctly sci-fi than horror. The only thing I didn't really buy was one character's progression to being a threat to the others.
Stir of Echoes (which is based on a novel by Richard Matheson) is underrated. Unfortunately, it had the bad luck to come out a few weeks after The Sixth Sense, which basically ate its lunch.
is a good film, but it's the one I like least on my original list. Personally, I thought The Others
(which came out a few years later) was better and a somewhat more "believable" take on a similar idea.