I don't know what it is, but you can try posting its picture on the Butterfly forum and see if you get a response there.
-Jmcat

Thank you. He is so huge, I am so curious as to what he will look like once he is a butterfly. It could be a monster butterfly!!

Thanks again! :)

Caterpillars of that size can be counted on turning into a moth, rather than a butterfly. In this case, the very pretty Imperial Moth. Since this guy was found crawling around, he was likely trying to find a place to pupate over the winter.

If you intend to keep him, put a few inches of dryish soil in a container, covered up with any kind of leaves. It won't eat at this stage, so you don't have to worry about that.

Put the container in your garage or other cool location and 'maybe' the moth will emerge in the spring.

Thanks for the ID. I found this same caterpillar munching on Pinus palustris In S.NJ. When I picked it off it gushed green goo out of it's back end onto my gloves!

Here is a link that might be useful: Imperial Moth Caterpillar