The Woodrow Wilson Foundation Leadership Program for Teachers 2000 Summer Biology Institute on Biodiversity lists these 10 common weeds:
Milkweed - An important nectar source for bees and other nectar seeking insects, and a larval food source for monarch butterflies and their relatives, as well as a variety of other herbivorous insects. It is not the most beautiful flower in the world, but it serves an important function in the environment.
Plantain - Current use of plantain is the commercially significant extraction of its mucilage – a carbohydrate fiber that is used in gentle laxatives. Mucilage also acts as an appetite suppressant and reduces intestinal absorption of fat and bile. It reduces LDL cholesterol and triglyceride levels in the blood. Plantain is commonly used as an astringent; its juice, when rubbed on an insect bite or bee sting, immediately sooths the area and begin the healing process. It can also stop poison ivy from blistering and itching if applied to the skin immediately after contact.
Red Clover - Studies are being done in the use of red clover for combating AIDS, diabetes and the increased cardiovascular risk associated with menopause. Red clover is a member of the legume (pea) family. These are a group of plants that are able to take nitrogen from the atmosphere and make it biologically available to other plants. Nitrogen fixation is of critical importance in protein production in plants and makes the legumes a critical player in agricultural planning.