That, my friend is a Tomato Horn Worm.
Cute as it may be (in my opinion) you must pick it off and squish it. It's a caterpillar that will absolutely decimate your plants with its aggressive appetite.

I think that it's the caterpillar of those cute little white butterflies that's always in the garden. Maybe someone can varify this? luke

Nope, those are cabbage loopers. The moth that the hornworm turns into is this one:

Here is a link that might be useful:

I was wrong.....It's the larve of the large 5 spotted hawk moth. I feel better now. luke

I picked 2 off one of my BW last weekend, saw another that wasp had laid eggs on so left it - the guy's still there today, and 2 little greenies got chewed. Big poopies - I don't know if there's another one on that plant, they're good at hiding. Don't see any signs on other plants (yet). Guess I gotta get a black light bulb for flashlight and go out at night.

Luke - those little white butterflies lay eggs that turn into little green inchworms that destroy cole crops - so check your cabbage, etc. if you have any planted.