I recently was reading through a food magazine and one of the hot items for this winter was greens. The author went into depth about several common ones and not so common ones. One that was left out of the article that got me thinking, was my lowly radishes that I have growing in my garden. I planted an heirloom variety of radish that produces a long tubular radish with very large leafy greens. Being the cheapo I am, I decided to give them a try. I chopped them up and put them in our salad and they were palatable, although a bit tough and hairy, but I got used to them. Now sauteed with some butter and garlic like you would spinach, THAT is where they shined. Wash them well, chop them up a bit and quickly saute them in a hot pan and you will likely not tell the difference between them and spinach. If you have a garden, the best part is they are free and be a lot more nutritious than the store bought spinach.
Additional types of greens to consider for this type of treatment would be beet greens, swiss chard greens and if you are very adventurous you may try broccoli and cauliflower, although I haven't yet.