That's a pretty broad question. Almost all plants, or some part of them, have some nutritional value. The number of poisonous ones is actually relatively small, but they can be rather deadly, so I wouldn't just go around eating things. All the fruits and vegetables you find in the markets are, of course, safe to eat. Most grasses are safe to eat. (Oats, barley, rice, corn, wheat, etc.) I would recommend some good books on the subject, such as "Stalking the Wild Asparagus" and others by Euel Gibbons.