It's a Tomato Hornworm. You are lucky(or not) that it isn't full of babies. You're going to want to go through and pick them all off and spray them with soapy water to kill them. They can decimate your plants. Keep your eye for them for the rest of the season too, because it may have already had it's babies.
RE: I came across this caterpillar in the garden yesterday, does anyone know which caterpillar