When I was growing up, in a landlocked city where no fresh clams could be found within a 50-mile radius, my mother made us a questionable version of pasta with clam sauce at least once every other week. It amounted to a bowl of spaghetti swimming in a mixture of garlic, chili, black pepper, bottled clam juice, and chopped clams out of a can.
It took a long time before I really became aware of fresh clams, and, though I've since fallen hard for the good stuff—whether baked, in miso soup, or in true pasta alle vongole—that spaghetti dish of my youth has maintained an undeniable hold on me. So every now and then, I give in to the urge to make it. It helps that it's incredibly quick and easy, and that I always have the necessary ingredients lying around: the spaghetti, the black pepper, pickled chilies, garlic, and, of course, bottles and bottles of clam juice.
My early and repeated exposure to bottled clam juice—usually made from the liquid used to steam fresh clams, strained and mixed with salt—is probably why I've always been surprised by people who express an aversion to it. Okay, so I can see how the name may be unappetizing to some people (given my love for clams, it sounds manifestly delicious to me). And sure, its sallow color may remind you of earwax or used dishwater or other...vaguely unpleasant things. But trust me—it's a versatile staple that's well worth keeping in your pantry.
Straight from the bottle, clam juice is a perfectly suitable replacement for fish stock, or fumet, in any recipe. It adds a marine baseline to soups, stews, and braises, similar to what fish stock would provide, and, because clams are naturally rich in glutamates, it has an inherent flavor-enhancing quality. Both Kenji and Daniel have recommended using clam juice in a number of recipes—as a liquid base for salmon chowder or clam chowder, as part of the poaching liquid for fish cooked à la nage, or to add extra shellfish flavor to shrimp fra diavolo.
Clam juice can also add flavor to the steaming liquid used for mussels, no matter what else you might want to spike the recipe with. (Of course, it's equally appropriate as a liquid for steaming clams.) It's effective at adding flavor in large part because it's relatively pure; as Kenji says, "Unlike most boxed broths and stocks, which often contain very little actual broth [especially beef broths], clam juice is literally just clam juice, water, and salt."
That added salt is one reason you have to be cautious when using the stuff. If you're using clam juice as stock, you'll want to take into account the extra salinity and either dilute with water or adjust your seasoning to accommodate it. The easiest way to judge the juice's saltiness? Set aside any qualms you may have, and take a quick nip from the bottle.
Which brings us to clam juice's other, equally interesting application: drinks. You may be familiar with Clamato, the spicy beverage of tomato juice concentrate spiked with clam juice, but you don't have to rely on a brand name to give your sangritas, Vampiras, or Bloody Marys a savory boost. Instead, make your own blend of clam juice, tomato juice, and spices, or try adding clam juice directly to your michelada as part of the umami-bomb component.
It can be helpful to think of clam juice as similar to dashi or fish sauce. No need to stop at seafood applications, since, in small amounts, it can add flavor without contributing any discernible fishiness at all. You might use it as part of the cooking liquid for polenta, as I sometimes do in place of plain water if I don't feel like making dashi (this is a particularly appropriate move if you're serving the polenta with clams). But don't limit yourself to our suggestions; once you start maintaining your own supply of clam juice and exploring what it can do, you're bound to find plenty of uses.