I'm scared of spiders bigger than a half-inch across... any smaller is fine. Heck, I pick up the mumping spiders around my house all the time, because they're so neat. But anything bigger freaks me out (especially tarantulas).
Several months ago, while on my computer, I noticed one of those huge, 6-inch long, blue centipedes running right towards my leg. I got up, and proceeded to use the chair to continually roll over it (though anybody who's ever tried to kill one of those huge suckers knows that it takes a lot more than that to stop them from moving). So, figuring that it wouldn't kill him, I had to put my weight on the chair and called my dad to get something to kill him.
About a month later, while playing GT3 (I sit on the carpet while playing), I again noticed a big, blue centipede running right to me, so I got up to go get something to hit it with. When I came back, it was gone. I was so scared the rest of the night...
And, several months before these two incidences, I was in my parent's closet to get something. They have one of those walk-in closets, and above the rods that hold the clothing are "shelves" that we use to hold suitcases and the like. So, anyway, I was using a short stepping ladder to reach the suitcases, and so my head was about 5 inches from the ceiling. Then, I noticed that there was a big blue centipede right above me. Oh boy did that scare the bejeezers out of me. I worked my way down the ladder, got a fly swatter, tried to hit it (scared of course that it would fall down on my face), but it escaped my swatting and hid behind a suitcase. Of course,I promptly decided that I was finished looking for whatever it is that I was trying to find, since I'd have to work in that area.
Scorpions scare me too... thank goodness the ones around here are small enough that you can just give them a good swat with a fly swatter and they'll be dead.