Of course there are ways to improvise around nearly everything when making drinks, but here are the basic tools I’ve invested in and find regular use for:
Jigger (first appearance: 002 – Old Fashioneds) – Jiggers come in various shapes, sizes, and materials, although as a standard-bearer I rely on a no-frills stainless steel version. The larger end gives you a 1.5 oz pour, equal to Whitfield’s definition of a jigger and also consistent with a standard shot at a bar nowadays, and the opposite end gives you half that (.75 oz). One thing to remember: in order to provide accurate measurements, you must fill the jigger to its very brim – so have a towel around to catch spare droplets unless you’ve got uncommonly steady hands.
Muddler (first appearance: 002 – Old Fashioneds) – Muddlers are most commonly used for fruit or herbs, to release oils and flavors into the drink, although today we’ll use it to crush and mix our “lumps” of sugar. When buying a muddler, you want to ensure it is made with unvarnished wood, since a stain or varnish will inevitably start to come off with repeated use (and where will it go? Into your drink!). My muddler comes from Vic Firth, originally a maker of drumsticks who has branched out into producing great wood cooking tools (in fact, my father enjoys the use of one of his pepper mills).
Barspoon (first appearance: 002 – Old Fashioneds) – you hardly need a dedicated spoon to make drinks, at least in my experience, but it’s still nice to have on hand (and the long handle comes in handy when stirring things in tall glasses or a shaker). My spoon doesn’t quite match Whitfield’s measurement of 1 barspoon = 1/2 tsp (it’s a bit bigger, as many American things have gotten in the past 70+ years), but it still comes in handy when you don’t feel the need to be overly fussy with small amounts.
Shaker/strainer/mixing glass (first appearance: 003 – Manhattans) – An invaluable tool to have on hand – the necessity of a good vessel to stir or shake ingredients with ice and strain the results can’t be understated. Shaking will give your drink a froth and make it more dilute (as the ice received quite a bit more abuse), whereas stirring keeps the spirits and flavors a bit more pristine. In either case, preparing a drink with ice and then straining it out is known as serving the drink “straight up.”