Oh god, I use so many. I've done a few treks where half of my sustenance came from what was around me. I'll name a few.
The most obvious friend of mine is Dandelion, I boil up mass quantities of the leaves, for greens. Then I fry up the rhizomes with bacon/other meat, or fish. You can dry the roots and use it to cut coffee with, and make it last longer. You can also use roots in a greater quantity to make a good cough medicine/stomach settler. Also, the flowers make a great wine. You can use every part, and it grows almost everywhere in North America.
Another particularly prominent one is the plantain leaf. You can boil it as a green. You can eat the seedstalks to get some fiber in your diet, not to mention if you eat 3 or 4 a day, you repel mosquitoes, as it makes your blood taste badly. You can take the bigger, broader leaves, and crush them on wounds and abrasions, and it helps them heal immensely. Make a compress on wounds (with Aloe Vera if you've got it) and it'll heal em in no time.
Clover is a great one in farm country. Grows everywhere, and eat enough of the flowers, with a cracker or two and some jerky, you've got yourself a great meal, and you can keep walking as you eat it! Just pick the flowers off the roadside as you walk and eat! The leaves are delicious as well. And you can use the flowers for an eye wash if your allergies are acting up.
Mint is a favorite of mine for seasoning foods, particularly fish. It's a SUPERB cough medicine, and also is great for belly ailments. If I had to pick one herb to have with me, and one herb only, it'd be mint. I've even eaten it as a green (mixed with others).
Cattail is my all-time favorite. It yields food ALL year. In the spring, the fresh young shoots are delicious eating, great fried with bacon or squirrel. Once they are tough, the roots/rhizomes are good eating too, but tougher to deal with, Gotta macerate them, dry them, and make a flour out of them. They can also be eaten like a starchy potato, if peeled well. Then, late summer, the pollen that gathers on the flowers is great mixed with flour for a delicious bannock. The roots can be dug all year, too, even in winter if your really going hungry. I've also heard of people boiling the brown flower heads like corn and eating them, never tried it though. Oh, and the sappy goop between the leaves at the base of the plant is great on wounds, it really heals them up nice. Put it on your face as a cleanser. And the leaves are great for weaving mats, I've also used the stem as a pipe reed on my corn cob pipe. You can build shelter with them, too. The local indians here in upstate new york revered this plant as a staple food, and used it for everything!
Burdock is good too, you can pull it up from the ground (it's tough to do, use a knife or a shovel) and the roots can be peeled and boiled. Sorta bland though.
Obviously wild berries/apples are awesome. Blackberry leaves can be used as a wash for wounds.
Less wild plants that I use: Oregano (a great disinfectant for wounds) Lavender (awesome for washing with, add some animal fat and some lavender to warm water if you don't have soap, smells good and keeps you clean) Bilberry (I grab them in pill form from grocery stores, and take them to aid night vision. It really works, I've got the eyes of a pilot)
If I think of more I'll post them! I've studied wild edibles and medicinals since I could read, I've always had a love for plants.