The caterpillar in the photo above is that of a White-marked Tussock Moth (Orgyia leucostigma). White-marked Tussock Moth caterpillars can be found on almost any woody species in eastern North America, but they are most commonly found eating apple, birch, black locust, cherry, elm, hackberry, hickory, oak, rose, willow, fir, hemlock, larch, spruce, and other conifer. The hairs of the caterpillar can cause an allergic reaction, so be careful if you see this one.
This is an American Dagger Moth (Acronicta americana) caterpillar that I found on the side of our house. This caterpillar can range in color from yellowish to white. They are most commonly found in forests and woodlands in eastern North America, and can be found feeding on alder, American hornbeam, ash, basswood, birch, box elder, chestnut, elm, hazel, hickory, horse chestnut, maple, oak, poplar, redbud, sycamore, walnut, willow, and other woody plants.
The IO Moth (Automeris io) caterpillar is a member of a group of caterpillars that have many-branched, poison-filled spines that give a sting that apparently feels like that of stinging nettle (but lasts longer). The IO is found in fields, woodlands, forests, and streambanks in eastern North America from southern Canada to Florida. They can be found feeding on aspen, birch, blackberry, cherry, clover, elm, hackberry, hibiscus, oak, poplar, sassafras, willow, wisteria, and grasses (even corn!).
And finally, my favorite caterpillar, the Spicebush Swallowtail (Papilio troilus). Spicebush Swallowtails are found throughout much of the eastern United States, most commonly in woodlands and swamps, but they can also be found in fields and gardens. They feed on sassafras and spicebush. This caterpillar is showing a form of mimicry. Its head looks like that of a snake, especially to a passing bird, who may then think twice before trying to make a lunch of this species.