These are kohlrabi. Kohlrabi are in the broccoli family; both the root and the leaves are edible.
So what do you *do* with them? Lots and lots; from Chow.com : "Cut into slices or wedges and add to Chinese stir-fry or Indian curry. Combine peeled kohlrabi with potato when making scalloped potatoes. Dip kohlrabi slices or sticks into tempura batter and deep-fry. Add shredded kohlrabi to coleslaw for extra crunch."
This is how I ate mine recently and they were delicious! Roasted Kohlrabi. Dice the peeled kohlrabi, toss with olive oil, garlic and salt, then roast in a 450-degree oven for about 30 to 35 minutes, stirring the cubes every five minutes once you've reached the 20 minute mark. Once they're done, serve immediately as is, or toss with your favorite vinegar.
Then I saved the leaves and steamed them in a stir fry!