As my palette develops to taste more and more nuances throughout different foods, I find myself in a continuing struggle to perfectly balance flavors in any given dish. The balance of acidity in food is an important one, and often underestimated by great home and professional chefs. The addition of vinegar (a worthwhile post on its own) or citrus juices is of the up most importance to round the flavors of more dishes then you assume. Now while citrus is a great addition to many things, and I feel naked if my home fridge doesn't have a few lemons and limes sometimes when trying to balance flavor vinegar is a much better bet. That being said the freshness that happens while finishing a dish with the juice of a lemon or lime is unmatched by any other product. I find that it will brighten the overall flavor in almost anything, rounding and highlighting everything else involved. It offers a crisp, clean, refreshing boost to soup, sauce, starches, salad greens, and even proteins. The acidic content of lemons and limes (limes have a higher acidity then lemons, and the highest of any natural ingredient available) can also cook a protein. Ceviche is a dish that is popular in many cuisines especially ones that are dependent on seafood. Usually raw prawns (squid, scallops, lobster, halibut, clams, and many more can be used) are chopped fine with a blend of peppers, onions, aromatics and salt and then allowed to soak in the lime or lemon juice. Depending on the size of the protein involved as little as a few minutes the acidity will begin to denature the protein structure. A quick warning, even though the risk of food borne illness is pretty slim here, it is important to buy good fish, from a trusted source, or when ordering at a restaurant use common sense. If you walk into a fake Mexican restaurant that looks like it hasn't had a customer in two months then I would stay away from shellfish in any form especially raw. Classically in Mexico ceviche it is almost always served with saltine crackers and a sort of spicy cocktail sauce condiment.
While we assume that citrus is a winter crop (I still get oranges every year in my Christmas stocking from my parents) citrus is a rotating crop and comes from all over South America, Mexico, Florida, and California as the seasons change. This helps them to have fairly stable pricing throughout the year and helps them to always be available. The availability of tangerines, mandarins, kumquats, and others is always late fall into spring. I am a big fan of lemons, but love a meyer lemon as well if you can get your hands on them, a bit sweeter and less aggressive. Blood oranges are another one that seem to have faded in and out of popularity a few times. They are a variety of orange that has orange skin speckled with red, and a magenta or maroon flesh depending on the variety. They aren't as sweet as most oranges but the juice and segments are an amazing color to offset color and add flavor to all sorts of things. Look for both of them January and February...ish.
My favorite thing in the world is to buy arugula and dress it with some really good olive oil, salt, and then just the juice of some lemon. Again showcasing the simplicity of seasoning and allowing the products to showcase themselves. Anytime I am cooking any sort of bean or taco dish I will small dice an onion and some cilantro and toss them with some salt and then toss it all with the juice of a lime. I always try to get it to meld for at least an hour or so, but the lime juice will take away most of the harshness from the raw onions, and you will end up with macerated deliciousness that you can eat with a spoon, as a condiment, or as a garnish for soups or beans. Almost any time I am going to serve a raw onion (sandwiches, salads, etc) I try to do this to soften the blow that can be the raw onion, making it easier to digest, and easier on your breath afterward. Lemons and limes can help your efforts to create cleaner flavors, don't overlook their importance or underestimate their abilities.