Harriet Tubman is great because she's the "Moses of her people."
She led of slaves to freedom along the Underground Railroad, a secret network of the safe houses where runaway slaves could stay on their journey north to freedom.
Also she became a leader in the abolitionist movement and during the civil war she was a spy with for the federal forces in South Carolina, as well as a nurse.
Harriet Tubman worked in the underground railroad for 8 year (1850 - 1858) which lead her to become famous for helping more than 300 slaves! She led them to freedom.