I'll second the swiss chard. It grew fast enough all winter long to outgrow the damage caused by my chickens feeding on it. I'll be planting a lot of that this fall. From all the ringing endorsements, I'll be looking for Kale seeds too!
Nice ID CompostQueen! Where do you live? That might make a difference. My girls really liked brussel sprouts, broccoli, carrot tops, radish tops, snow peas, radicchio, endive, all different kinds of lettuce, bok choy and I'm sure I'm forgetting a lot. I don't have space to grow cover crops...