i find the toe of a courier down the door panel gets a drivers attention, though i've heard twatting the mirror off with yer hand is very effective also
Stibel Magnum (from Nippy Norman's - link at top of page). Fits straight in with no mods and is VERY loud
Not sure if you get it in the States but over here the 'youth' will often walk very slowly across the road forcing motorists to slow down in order to avoid hitting them
Had a young guy do it to me yesterday, rode up beside him and let fire with the horn - oh how I laughed as he shot 30ft into the air with a look of shear terror on his face