collard greens - these guys need long cooking. You can also stuff them as you would make stuffed cabbage. Vivian Howard's Deep Run Roots has a great collards section as does Six Seasons, mentioned above.
fennel - I think of fennel more as a stalky bulb than a green, though it has plenty of feathery fronds. It has an anise flavor that's muted by cooking. Raw, it's delicious in salads with oranges and other citrus. As mentioned above, Tenderheart has a great fennel section.
chard, beet greens, turnip greens - I put all these in the "cooking greens" category along with mustard greens. Sneak them in anywhere you put cooked spinach
kale - I much prefer the dark green lacinato kale (aka dinosaur kale, Tuscan kale, cavolo nero) to the common curly kale. Takes longer to cook than the greens above but can generally be used in the same way. Baby kale is available in my local farmers markets and can be used fresh in salads or cooked.
watercress, arugula, endive - I generally eat all of these fresh, though endive (which comes in many varieties) is lovely braised. See here for more endive ideas.
