Mmm, horror. Never going to be the front-runner in the popularity contests, hard to run effectively, has to be custom-tailored to the group more than almost any other genre. I loves me some horror gaming.
There's always the granddaddy, Call of Cthulhu, and, of course, the d20 edition. Nothing like watching the investigators slowly going insane when, in any other game, they would be "winning." Chill's an old favorite, even if it's a little long in the tooth now. D20 Modern is easy enough to adapt to a good old-fashioned monster hunting campaign, but it can be hard to make the players afraid when their characters are meant to be heroic, so it's maybe better for a monster hunting campaign. For more of the urban fantasy/Hellraiser style of game, there used to be Kult, and now there's Unknown Armies, but are there any other good ones out there?
And, of course, on the other side, there's all of the World of Darkness, old and new, and... um... there was Night Life in the early 90s. In Nomine is vaguely horror, depending on how it's run. I know I'm missing some here, so I'll let those who like the games where you actually play the creepy crawlies instead of setting fire to them leap on in.