Horror, you say? Well, I don't want to discourage anyone, but those games are hardly a top notch stories. If we're already on off-topic department (cause it's not about N900 games, and should rather land in off-topic), I would like to recommend something absolutely awesome, instead.

It is horror adventure, but despite many macabre moments, the story itself is driving point. Not a thing that tries to scare you by a monster jumping out and doing "woo!", oh no. in fact, they're not trying to scare you at all, instead, characters are very relatable. Despite terrific events, you're just to immersed in the story, to have time for being scarred. See a teaser:

The Cat Lady
http://www.youtube.com/watch?v=1P3o9LZJwrY

While made by one person (mainly), it's one of the best games I've ever played in my life, and I speak about it at the same breath with things like The Witcher or Planescape: Torment.

From technical point of view, it's using FOSS engine (AGS), but game itself isn't FOSS, it's a "fair play" proprietary one (no DRM). I'm currently trying to contact author about possibility of running it on ARM linux environment. Playing it on N900 would be a nice feat (but don't hold your breath, I'm not sure if it will be ever possible).

/Estel