Individually wrapped, A&W root beer barrels -- packaged and distributed by Brach's candies, and sold at many fine retail outlets both here and abroad. My current source is the bulk candy kiosks at Publix supermarkets, but most stores are phasing these particular candies out, and I'm somewhat upset about this. The only acceptable alternative are Reed's root beer rolls (individually wrapped lifesaver-like root beer thingys). But it's a noticably different root beer experience that just doesn't satisfy the way that Brach's A&Ws do.
Not only are they quite addictive, and a great ice breaker, but I've found them to be instrumental in diffusing imminently violent situations.