"Natural":
Carrot Seed
Juniper (Juniperus communis)
Marjoram (Origanum majorana)
Wormwood
Mugwort (Artemisia herba alba)
Pennyroyal (Hedeoma pulegioides) or (Mentha pulegiumv) See: Pennyroyal Tea
Sage

Man-made hormonal:
RU-486

For actual medical procedures, see Abortion Methods.

Further Reading:
Conctraception and Abortion from the Ancient World to the Renaissance. 1992
Eve's Herbs: a History of Contraception and Abortion in the West. Harvard, 1997