Burdock is a common plant you've probably scene along roads. It's roots are particularly useful for purifying the blood. It can also help with skin problems (acne, eczema) and strengthens the kidneys.
Chickweed
I'm not sure chickweed was neglected when society decided which plants to include in the gardens. It's an attractive plant, though a bit straggly. It has a mild flavor which I've used in salads. And it's easy to grow. It's identified as a soothing herb, used both internally and externally. It's good for skin problems, inflammation, ulcers and lung problems. Because it's such a mild plant, it can be taken in larger quantities: up to four cups per day.
Cleavers
You've probably passed cleavers often without realizing what this valuable plant is. It's highly invasive and tends to grab as your socks as you walk by. As an infusion it supports the lymph system and so is a remedy for cystitis. It's useful for skin diseases and the juice is a diuretic. Traditional medicine recommends it for dropsy and it's been explored for anit-tumor properties, though research hasn't produced enough evidence for it to be considered as a pharmaceutical.
Colt's Foot
Another common plant that's often overlooked, Colt's Foot was important in traditional medicine. In fact, you will find it a common ingredient in over-the-counter cough syrups as an expectorant. It also supports the bronchial tubes and has demulcent properties, meaning it provides a soothing coating over irritated cells. It's also recommended for asthma.
Comfrey
I've encourage wild comfrey to grow in my garden with some success. You need to be careful in identifying it, however, since in its early stages it looks a lot like foxglove (the basis for digitalis). It's a large plant and pleasing to the eye. Used for its demulcent properties, it was also known as knitbone helps heal broken bones. It's also good for treating skin problems and to heal wounds. An ointment can be made by letting the leaves and roots decoct in hot wax.
Corn Feverfew, Mayweed, Wild Chamomile
Just like its cousin, the wild chamomile is a nerve relaxant and relieves gastro-intestinal problems. It is also an anti-spasmodic and anti-allergic. It's good in lotions and an excellent wash for cuts and bruises. You can identify it by it's chamomile-like smell.
Dandelion
Yep, that noxious weed that we're struggle to get rid of, was actually once considered so valuable the first Europeans brought them with them. Recommended for fevers and inflammation, the roots are highly recommend to support liver function. Used as a poultice, they help remove toxins from the skin. Think about that the next time you yank one out of the lawn.
Daisy (Little)
This is that little daisy that grows in your lawn. Also known as bruisewort, the leaves can be cruised and used to sooth wounds and relieve skin diseases.
Eyebright
If you find eyebright, you'll have an herb that's become very popular. Most herbal dispensaries offer it. As the name suggests, it's used for inflammation of the eye. It is also recommended in addition to fennel.
Horehound (white)
Horehound is an expectorant and often found in cough remedies. Horehound syrup is used to treat chills, asthma and lung problems. As a hot tea, it can also help reduce fevers and relieve constipation.
Horsetail
When I lived in Seattle, these were very common in the woods. Horsetail is so useful you can purchase it in herb dispensaries. It's an astringent, a diuretic and good for inflammation. It supports the urinary tract and is an aid for incontinence. It may also be used as a gargle for oral infections. Because of it's rich silica content, it can be used to grow long nails. It also helps acne and eczema.
Houseleek
These beautiful garden plants share many of the same virtues with aloe vera. The sap is astringent and cooling and can be used as a cold poultice for burns and stings. When applied to the head it can also relieve headaches.