Are there any important substances produced by a placenta in addition to hormones?
The placenta is the organ that connects the fetus to the uterus and serves as the direct conduit between the mother and the fetus. The only real chemicals the placenta produces are hormones as you mentioned including progesterone, estrogen, human placental lactogen (hPL), and human chorionic gonodotropin (hCG). Progesterone helps maintain the uterine lining. Estrogen is the female hormone and it in conjunction with hPL helps prepare the mother for lactation after the birth. hCG helps prevent the endometrial lining from being ejected through menstruation and allows the early pregnancy to take hold.
Although the placenta does not produce other chemicals besides hormones, it does help in other ways. It serves as the blood flow conduit between fetus and mother. It helps filter out waste products and provides nutrition from the mother's bloodstream to the fetus. It also helps promote the oxygen/carbon dioxide gas exchange for the fetus's blood before the lungs develop.