This is an Imperial Moth caterpillar. They come in different colors (more often green than brown), but the four horns near the head are distinctive.

If you see a giant, plump caterpillar, it is probably a member of the silkworm moth family. This Cecropia caterpillar (left) will turn into North America's largest moth.

Polyphemus Moth caterpillar. These giant caterpillars are bright flourescent green. They look similar to Luna Moth caterpillars (below), with some minor differences.

Luna Moth caterpillar. The adults of this species are among the most beautiful animals in all creation. Nature can be beautiful ... but it can also be heartbreaking. This unfortunate caterpillar has several parasitoid eggs on its head. A large number of butterfly and moth caterpillars fall victim to parasitoid wasps. 

This is a group of early instar Io moth caterpillars. Later instars tend to be green, with a red stripe along the side. Io moth caterpillars can sting, so be careful about handling them! I found this group feeding on a leaf at Huffman Prairie.