Rum's Friends & Relations
There are a few unusual liquids you might want to know about that go extraordinarily well with rum.
Falernum: a spiced syrup with elusive flavors of almond, vanilla, lime, allspice, and ginger. A touch of Falernum provides complexity and mouthfeel to virtually any rum cocktail, and it is directly specified by many recipes. Falernum is domestically available from a few different brands, notably Fee Brothers West Indies Style Falernum (non-alcoholic) and John D. Taylor's Velvet Falernum (slightly alcoholic).
Sugar cane syrup: there are lots of bottles of sugar syrup on the market that purport to be "sugar cane syrup" but are really just refined sugar dissolved in water. Real sugar cane syrup isn't just sweet; it has the pronounced vegetal flavor of raw sugar cane juice! That flavor makes real sugar cane syrup a much more interesting sweetener in cocktails (even ones without rum) and other culinary endeavors. How do you know which syrups to buy? As far as we know, the only one that's available in the U.S. as of this writing is Petite Canne Sugar Cane Syrup.
Lemon Hart 151-proof Demerara rum: most overproof rum is white rum (a harsh, unaged moonshine-like spirit). Lemon Hart's 151-proof Demerara rum is deliciously different and nearly the only overproof rum that is relevant to cocktail-making. Tip: even if the recipe doesn't specify it, many rum cocktails get a little lift if you add a small float of this spirit.
Swedish Punsch: a spiced, smoky rum liqueur (the rum equivalent of Drambuie) produced in Sweden and Denmark by various companies, with a base of Batavia Arrack. Traditionally incorporated into toddies or punches, Swedish Punsch has also snuck into various cocktail recipes, mosly involving rum. Tip: if you stir a small spoonful of Swedish punsch into a glass of so-so aged rum, the flavor of the rum is greatly improved! Swedish Punsch is not currently imported to the United States but is fairly easy to come by in Copenhagen or Stockholm. You can also try mail ordering through the Internet.
Pimento Dram (a.k.a. Berry Hill): you can work a little of this allspice liqueur, produced by Wray & Nephew and sold exclusively in Jamaica, into virtually any cocktail and not regret it. This inexpensive liqueur is sublime, but to get it, you have to go to Jamaica and hit the local shops (you won't find it in a resort.)