Wild lettuce
Wild lettuce, also known as bitter lettuce or opium lettuce, (Lactuca virosa) is an edible plant that grows abundantly in the wild. As the common name opium lettuce implies, it has has medicinal properties moderately akin to opium. A dark, putty-like extract of the plant can be made the same way as done with opium poppy that can used similarly, although being much weaker (as well as not quite providing a "high" like opium does) to treat minor aches and pains as well as induce sleep.
Identification
Wild lettuce is fairly easy to identify for beginners and is a staple plant to look for while foraging.
Food
Wild lettuce can be used just as regular lettuces are used, however being slightly more bitter. Eating large quantities of the leaves can have similar medicinal effects as taking an extract.