Dark rum floated over ginger beer with fresh lime — the trademarked Bermuda classic.
The Dark & Stormy is one of the few cocktails with a registered trademark — Gosling Brothers of Bermuda trademarked the name and require their Black Seal rum to be used in any 'official' Dark & Stormy. The drink originated in Bermuda when sailors mixed Black Seal rum with locally made ginger beer.
The float technique is crucial and often done wrong. The dark rum is denser than the ginger beer and naturally wants to sink — floating it over the back of a spoon keeps it on top until the drinker stirs or sips it down. This creates the 'dark and stormy' appearance of dark clouds over a ginger-coloured sea.
Replace rum with bourbon and use the same technique. A different but related long drink.
Replace rum with reposado tequila. The ginger and lime work beautifully with the agave's earthiness.
Add ½ oz falernum (spiced almond-lime syrup) to the ginger beer before floating the rum. Adds complexity and a slight sweetness.