Spiders are apparently a significant symbol in dreams: a spider means good luck, but if you kill the spider it can mean bad luck; a spider means you are feeling like an outsider; a spider is a symbol of feminine power; a spider spinning a web means you will soon be rewarded for your hard work, or it means that you are entangled in your current situation.
In my dreams, however, spiders mean one thing: the following day I am going to see a spider.
There was a fat ugly orange spider in my dream the other night, and since I dislike spiders, I killed it (ooo, bad luck!). The next day I was searching in the deck box for my goggles and came across a black widow. I didn't kill it (it was outside and I was in a hurry and had bare feet and no death spray). I guess it was my fault for looking in the deck box, the known home of several probable spider colonies, but I love the coincidence all the same.