Certain Euphorbiaceae, specially of the genus Hevea, were initially responsible for most of industrial latex, used to produce rubber. Now, most of the industrial latex is artificial, but natural rubber from the latex of Hevea brasiliensis is still used as an alternative to many products, specially clothing.
Latex is a natural deffence against predators, and that´s the reason why the latex of most plants are incredibly poisonous, toxic, or skin irritant. But there are a few exceptions:
The Milk Tree (Couma macrocarpa, family Apocynaceae) has an edible, tasteful, nutritious, sweet latex that can be drank pure or mixed with water or juices. The latex of Amapá (several species under this name) is medicinal, and also can be taken pure in small quantities. The latex from unripen Papaya fruits have enzymes that are used to soften meat. Chiclé, the basis for chewing gums, is obtained from the latex of several species of Manilkara, family Sapotaceae.