I got this idea, from here. It's one of my very favorite sights! These were originally Christmas ornaments from the dollar store. They came in packages of 8. They are styrofoam, but whatever is on top of them (glittery paint), stopped the spray paint from melting the styrofoam. They started out white and we spray painted them gray, because well, it was what we had :). You could also paint them with acrylic or tempra paint. We cut pipe cleaners and stuffed them into the styrofoam to make the legs. Add some googly eye's and you've got a cute collection of spiders.
This is a craft that is doable for preschool, but in small groups. I had to really help the two girls, who are both 3. My 6-year-old did great. This would be a great craft for a school party.