Milkweed (Asclepias syriaca) can be eaten at several stages. The best are the shoots before they are 4 inches in height, and the seed pods before they are 3 inches in length. To eliminate the toxins in Milkweed, it must be boiled in water twice, each time pouring of the water and replacing with hot water. Best served like Spinach.
Dandelion (Taraxacum officinale) leaves are good to eat. Some like the leaves at any age, but I think the older leaves are a bit too bitter. The leaves can be eaten raw,or cooked like spinach. If you've ever heard of Dandelion Wine, it is made from the flowers of the Dandelion, I won't go into the recipe for that just yet, if you want it there are some good books at the library that will tell you how.
St. John's Wort (Hypericum perforatum) is used as a medicinal herb. It is used mainly as an anti-depressant, but also as a sedative. Maybe just being sedated a bit can make you happy I suppose. To use it dry the whole above ground part of the plant, then remove as many of the larger stems as you can (leaving some in won't hurt you). Now there are two forms, people take it powdered in capsules, and steeped as a tea. I find that it works rather well for me, I use the standardized capsules. But heed the warning, it makes some people very sensitive to the sun, so they sunburn easier. Now, I won't give you the dosage, again, go get a book, read up on this stuff!
The Groundnut (Apios americana), now we've all read about how one of the foods that helped the Pilgrims make it through there first winter here was Groundnuts. Well I'm here to tell you that they are not bad eating at all! I really love the taste of Groundnuts. It can be a little irritating sometimes to go forage enough of them, but I think well worth it. The procedure is simple, go dig some up, cook them like potatoes (any way you can make potatoes, you can make groundnuts the same way).