The cabbage butterfly, Pieris rapae (Linnaeus)http://en.wikipedia.org/wiki/Cabbage_Butterfly
This is a female. The way to know this is by the two black spots on the fore-wings. The male does not have these spots. However both have the black tips on the fore-wings.
I actually find these rather fun to see in the garden. I like them actually. However like the name implies they love cabbage and broccoli as well. The adult is not a threat in them selves. However the caterpillar can cause great crop damage in some areas.
My Dad used to give me money as a kid if I killed the little green guys (caterpillars) when I found them, hence I was a great hunter. To-day I leave them alone and let nature do it for me. I do not grow cabbage either so that may well be the reason I do not go after them !!!!! Besides Terry would kill me. She will now any way after she reads this !!!!
This one is sitting on the leaf of the Elephant Ear Plant featured just below this picture. It seemed so small on the huge leaf (1 meter X 1/2 meter) big.