I was looking through Instructables.com and came across this:
I used to make this when I was really young. You just fold a piece of paper into that shape and flick it really hard. It makes really loud noises and scare the wits out of anybody nearby. Such fun!
And now for a bit of physics while we are at it: How does this work?
Basically, when the centre-fold of the paper gets flicked out, it moves at a very fast rate and pushes the air in front of it away quickly. This creates a shock wave which can be really loud.