|a Persian lime|
As far as I'm concerned, limes are a necessary ingredient to good guacamole, as is cilantro. (Limes and cilantro have an affinity for each other, and are often found together in marinades.) I use limes in fish recipes, with chicken, and in numerous warm-weather drinks. One average lime creates about a teaspoon of juice.
Limes have been around for a long time. British sailors used them to combat scurvy, which is why they were sometimes called Limeys. They are available year round, although the height of the season is from now through October. Americans mainly use two types of limes: Persian limes and Key limes. Persian limes are sweeter and larger and much easier to find.
One odd thing to beware of with limes is that they have a chemical that can cause a bad skin condition in some people. The juice alone doesn't cause the problem—it's the combination of sunlight and lime that can make some folks break out in a blistery rash. So if you make a recipe using many limes, wash up carefully and use sunscreen for the next day or so.