The good old air horn. It has so many uses. On your boat as a USCG approved safety device. At your favorite sporting event. Think baseball and football, not golf or tennis. And my favorite use, the Air Horn Prank. Videos of air horn pranks after the break.
Enough about the uses, why this air horn? The Shoreline Pump Air Horn ($17) is the one to buy because it is refillable. You simply pump the handle at the end of the horn to refill the air chamber. And, it blasts at 120 decibels and can be heard 1 mile away.