Elena Glinskaya ruled what would become Russia as regent for her son, wisely and ably in fact, until her life and reign were brought to brutal ends.
Amalasuntha ruled Rome after the Roman Empire's fall. She is known for being a wise leader of the Ostrogoths, first as regent, then as the ruling monarch.
Alice Perrers was quite the ambitious mistress, though we have to have some sympathy for her—she has been hated in her time and ever since.
Anne Hyde nearly became Queen of England but died before her chance to. She was a commoner who, through good luck, became the mother of Queens Mary II and Anne.