You can buy radishes any time of the year, of course, usually overgrown, coarse, stripped of their leaves and thrown in a plastic bag. Right now, though, you can find the beauties of spring, fresh, in bunches.
In reality, there are two main radish crops, one in spring and one in fall. And traditionally, at least, they're completely different ingredients. Winter radishes include daikon and black radishes -- they're big, sturdy, often extremely strong-flavored, and long-keeping, more like turnips than their springtime cousins.
Spring-summer radishes, however, are usually small. They may be the familiar pink or red and round but they can also be also purple, white, or other shades, and they may be long instead of round. Abundant and at their best right now, they are, in general, mild enough to eat right off the leaves (which, for highest quality, should be still attached when you buy them). Rinse the bulbs well -- they spend most of their lifespan submerged in soil, so they might even require a little scrubbing -- and slice, chop, or grate them if you like.
When fresh, crisp, and young -- the longer they grow, the sharper they become -- radishes are delicious served whole (you can eat the stem end, too). If you're interested in real simplicity, serve radishes with salt and buttered good bread (or toast), or with a bowl of good olive oil to which you've added some salt and pepper -- they're outrageously good this way.
There are of course more "complicated" things you can do with radishes, but none of them gets too serious. I've made plenty of radish salads over the years -- one of the simplest involves only chopped radishes, onion, lime and lemon juice, parsley, salt, and pepper -- and I never tire of them, because there are so many seasonings that complement their distinct flavor. They become even crisper if you slice or chop them, then sprinkle with salt and let them sit for 10 to 15 minutes before rinsing, draining, and tossing with other ingredients.
My new favorite radish salad is this Thai-style combination, which features nearly all of that cuisine's greatest hits: fish sauce (also called nam pla, and now available in most supermarkets), fresh chiles, chopped peanuts, and fresh cilantro. It's almost a pickle, really, especially if you salt the radishes first and then let the salad sit in the fridge for a couple of hours, and it's a perfect accompaniment to noodles or stir-fries. Incidentally, it's also nearly fat-free, incredible when you consider the intensity of its flavor.
But radishes are fantastic cooked, too -- if you've only had them raw, do yourself a favor by slicing a few and sautéing them in butter until they're crisp-tender, or include them in any stewed or braised dish in which you'd use other root vegetables. And you really ought to try this braise-and-glaze recipe I learned from my friend Jean-Georges Vongerichten: It is obscenely easy, requires no fancy ingredients or techniques (you don't even have to cut up the radishes), and will make a convert of anyone who thinks cooked radishes sound weird.