The tomato hornworm is the larval stage of the Five-spotted Hawk Moth. Both stages of this insect are formidable to see, but the hornworm is not a welcome sight in the vegetable garden. Their preferred food is the leaves of tomato and pepper plants. Since they blend in so well with the foliage, you may not notice them until you start to see damage.
Because the hornworm is so large (about the size of a pinkie finger), the easiest way to get rid of him is to simply remove him from the plant and dispose of him. You may be lucky enough to attract parasitic wasps to your garden who will do the job for you. Parasitic wasps won't hurt you, but take a look at the next photo to see what they'll do to this guy.