I’ve always had at least one of these on hand over the last four years. For small amounts of liquid ingredients in recipes, like vanilla extract and soy sauce, it’s much easier than trying to fill a measuring spoon to the rim. For mixed drinks, it’s more precise than a jigger, albeit a bit slower. If nothing else, it makes a perfectly serviceable (and somewhat geeky) shot glass.
The measurement markings are in fluid ounces, milliliters, teaspoons, and tablespoons. A few different companies make them, and while they’re available online for $3-$5 a pop, I’ve seen them cheaper in brick-and-mortar stores like Bed Bath & Beyond and Target.
[Note: If you're feeling artsy, you can amend any old shot glass with a sharpie and a traditional set of measuring spoons into one of the measuring varieties. -- OH]