Far from the shrink-wrapped iceberg lettuce that used to populate grocery stores, most of us now face a choice of lettuce varieties and salad greens at the market – particularly if we shop at farmers markets.
Lettuces in general are cool weather crops, at their best in spring and early summer before high heats and long days make them bolt and turn bitter. Look for lettuce year-round in ultra-temperate climates, fall and spring in mainly temperate areas, and in the late spring through the summer months in cooler climates.
Also see How to Clean Salad Greens.

Photo © Molly WatsonArugula (a.k.a. rocket) has long, spiked, dark green leaves and a peppery flavor. Wild-harvested arugula is the most pungent (look for it at farmers markets and local foods co-ops). Cultivated arugula is widely available and varies greatly in strength of flavor. In general, larger leaves tend to be stronger tasting, but if pungency is a concern, be sure to taste the batch before using. Use arugula alone to stand up to tangy dressings and bold flavors, or mixed with other lettuces as an accent note.
Butter or Boston Lettuce
Butter lettuce is commonly available. It is a crisphead lettuce, meaning its leaves form a compact head as it grows--although its head is much less compact than iceberg lettuce. Butter lettuce has a tender texture and large, cupped leaves that work beautifully in salads, especially with delicate dressings. Look for pale green and red-tinged varieties.Little Gems

Photo © Molly WatsonLittle Gem lettuce is soft with just a hint of crunch. The delicate flavor is well suited to light vinaigrettes and lemon-y dressings.
Mache
Mache, also known as corn salad or lamb's lettuce, comes in lovely little rosettes of dark green leaves attached in groups of 4 or 5 at the roots. It has a bit more body than many lettuces and mixes well with other vegetables. It requires extra care when cleaning, since sand and grit tend to gather in nub of roots holding each rosette together. Give it a few extra swishes in the water to get them clean.Mesclun

Photo © Molly WatsonMesclun means "mixed" in Provencal and is traditionally composed of several varieties of wild-harvested, young greens. Most mesclun sold today is cultivated--planted as beds of mixed lettuce seeds harvested when the leaves reach the desired size of 3 to 6 inches). Look for mixes that contain young, sweet leaves from a variety of tender lettuces, maybe a bit of curly endive for texture, some peppery watercress or arugula for bite, and maybe a few herbs. Some farms and markets sell special "spicy" mixtures that have more arugula, watercress, mezzula, and mustard leaves.
Mizuna

Photo © Molly WatsonMizuna is an Asian variety of mustard greens. It has spiky dark green leaves that have a surprisingly delicate texture and delightfully peppery, even spicy kick.
Oak Leaf Lettuce
There are several varieties of oak leaf lettuce – green, red, bronze – but they are all loose-leaf lettuces, meaning the leaves stay loose and attached only at the base as they grow instead of forming tight, compact heads like iceberg lettuce or cabbage. They make excellent salads and work with a wide range of dressings. Discard the external leaves if they are damaged or wilted. If working with small heads, use the leaves whole. Larger leaves can be torn into bite-sized pieces when cleaning.Romaine Hearts

Photo © Molly WatsonRomaine lettuce is hale and hearty. Its crunchy texture can stand up to any dressing - from a light gingery vinaigrette to a full-blown heavy
Blue Cheese Dressing.
Watercress

Photo © Molly WatsonWatercress has a bright, peppery flavor prized for salads and gently "wilted" preparations. It grows wild in streams in Northern America and Europe, but is easily cultivated with the right irrigation. Much cultivated "watercress" is actually garden cress, which has slightly less bite and crunch than its watercress cousin. Whatever cress we're talking about, they're all members of the mustard family. The older they get – either in the ground or after being harvested – the sharper their flavor becomes. Use cress as soon as possible, removing any yellowed or wilted leaves. Tender stalks and roots are perfectly edible along with the dark green leaves.