If we're counting freckles as birthmarks, the answer is everywhere. Face, back, shoulders, arms, a few small and not very noticeable ones on my chest. I don't normally consider them birthmarks, but really, what else would you call them?
My husband has a fairly large, but light colored splotch right in the middle of his back. The first time I noticed it (due to the color, shape, location, and the fact that he had just stepped out of the shower) I thought he had missed a spot while washing.