What scares me?
Spiders:
Spiders, for every reason mentioned above by Henry. That unnatural precision with which they move is horrendous. To add to the fear, I was invenomated by a black widow before (note: not bitten). I was splitting firewood, and the wood blocks from this particular tree had become home to a black widow whose young had hatched and set up a colony. I smushed a few of the buggers here and there, when I saw them, but one managed to crawl its way onto my arm. I saw it, and squashed it while squealing like a little girl, and happened to somehow smash some venom into an open cut. The next day, my lower arm swelled to twice normal size. Luckilly there were no major side effects; I didn't bother going to have it treated, even.
Rotting?:
Someone mentioned gangrene, and watching parts of your body die and rot away. While not on a massive scale, one time I developed an infection in a cut on my finger after an auto accident. Overnight, the end of my finger swelled to three times its normal size and developed a most unpleasent smell, not unlike that of a dumpster filled with week-old organic matter. Had to go to the doctor immediately, where they cut and scraped out all the dead flesh, leaving me with a nasty scar. So, while rotting away is unpleasent, it isn't scary to me, at least when it's on a small scale.
Ghosts:
For many years, a ghost had lived in our house (it moved out several years ago, and we haven't seen it since then). It always creeped me out to see its shadow sliding along the walls, as it moved down the hall to the bedrooms or up to the kitchen. It was a benign being, though; not only did it not do anything harmful, it chose to not interfere in any way, good or bad.
I did have a bad experience with another member of the spirit world, though. It still terrifies me to recall this tale, and my hands are shaking as I type this response. I apologize, but I can't bring myself to go too deep into detail, but suffice to say that I was "attacked" (I guess you could call it that) by a ghostly being, who passed a part of itself through my chest, over my heart, and caused me to experience the most bone-chilling cold I have ever felt. To this day, I feel its presence inside of me, a lancing pain in the part of my heart that it touched. Spiders might give me the willies, but this "ghost," or whatever it is, is the only thing that I have ever been truly afraid of. (heh, it took me a twenty minutes to type that paragraph, but I got through it. ::does a happy dance:: )
I guess for a game, you could have almost anything on the ceiling... maybe on a really high ceiling that's just out of all weapons' range. *shivers* I'm gonna get no sleep for days after reading this thread. 
