Taraxacum officinale, the Common Dandelion (often simply called "dandelion")
Quote from wikipedia:
While the dandelion is considered a weed by most gardeners and lawn owners, the plant has several culinary uses. The specific name officinalis refers to its value as a medicinal herb, and is derived from the word opificina, later officina, meaning a workshop or pharmacy. The flowers are used to make dandelion wine, the greens are used in salads, the roots have been used to make a coffee-like drink and the plant was used by Native Americans as a food and medicine.  Dandelions are grown commercially on a small scale as a leaf vegetable. The leaves (called dandelion greens) can be eaten cooked or raw in various forms, such as in soup or salad. They are probably closest in character to mustard greens. Usually the young leaves and unopened buds are eaten raw in salads, while older leaves are cooked. Raw leaves have a slightly bitter taste. Dandelion salad is often accompanied with hard boiled eggs. The leaves are high in vitamin A, vitamin C and iron, carrying more iron and calcium than spinach. Dandelion flowers can be used to make dandelion wine, for which there are many recipes. It has also been used in a saison ale called Pissenlit (literally "wet the bed" in French) made by Brasserie Fantôme in Belgium. Another recipe using the plant is dandelion flower jam. Ground roasted dandelion root can be used as a coffee substitute. In Silesia and also other parts of Poland and world, dandelion flowers are used to make a honey substitute syrup with added lemon (so-called May-honey). This "honey" is believed to have a medicinal value, in particular against liver problems. Dandelion root is a registered drug in Canada, sold principally as a diuretic. A leaf decoction can be drunk to "purify the blood", for the treatment of anemia, jaundice, and also for nervousness. A hepatoprotective effect of chemicals extracted from dandelion root has been reported. Drunk before meals, dandelion root coffee is claimed to stimulate digestive functions and function as a liver tonic. "Dandelion and Burdock" is a soft drink that has long been popular in the United Kingdom with authentic recipes sold by health food shops. It is unclear whether cheaper supermarket versions actually contain extracts of either plant. The milky latex has been used as a mosquito repellent; the milk has also been used to treat warts, as a folk remedy. Yellow or green dye colours can be obtained from the flowers but little colour can be obtained from the roots of the plant. T. officinale is food for the caterpillars of several Lepidoptera (butterflies and moths), such as the tortrix moth Celypha rufana. See also List of Lepidoptera that feed on dandelions.