Scary Yahoo Prank
Basically, you type in this link that will take you to a "normal" yahoo page. When you are there, let the person you are about to prank, type in something that they want to search. When they press websearch, they will get scared with a scary message and a face pop up. (make sure volume is up)
Link : http://www.scaryforkids.com/pics/Yahoo-Search.htm
Damien: Have fun with this one. I wonder when a Google version will come up ^_^.