I bought this little box a while back that when opened has a fake spider that pops out of the lid. If you know anybody who is scared of spiders then this is something you must get in order to scare the hell out of them.
Here are a couple of videos I made of me pranking a couple of friends. It doesn't work so well on people who are not scared of spiders, although it probably will make them jump. Just go find someone who has an absolute huge phobia of spiders.